Several physiological factors or conditions which were conceivably capable of modifying the basal heat production of doves and pigeons have been studied and are found to influence the values obtained from these animals. The elimination or the control of these factors is essential to success in measuring such differences as may exist in the basal metabolism of the sexes, of different races, and of different hybrids, among these birds. A statement is made concerning the special nature and history of the wide variety of races (24 of doves; 13 of common pigeons) and hybrids upon which these and current studies are based. The dove and the pigeon, by their accepted classification, differ more than the rat and the mouse; and the distinctness of at least some of the many races of each of these species makes it necessary to treat and tabulate data with full recognition that they are different races or strains. More than a thousand birds of known pedigree, maintained under excellent conditions, have been continuously available for this study. Autopsies made usually soon after our metabolism measurements enable us to eliminate from consideration all values obtained on diseased birds. Our study of the time relations of the respiratory quotient shows that a 24-hour period following the taking of standard amounts of food insures essentially fasting quotients (about 0.72). It is not safe to begin metabolism experiments prior to the twenty-fourth hour, if one is to assume a fasting respiratory quotient and calculate the heat production from the carbon dioxide production only. The fasting period should begin near the end of the daylight hours—immediately following the hours of normal feeding. At this time the crops of ring doves should be provided (hand-fed, when necessary) with 5 grams of grain; those of common pigeons with 10 grams, to which water is added. In this way a favorable stage of fasting will be obtained 24 hours later, and the end of the fast then coincides with the most favorable time (night) for making metabolism measurements. Some birds not hand-fed at the supposed beginning of a fasting period will be measured in a fast actually much longer than 24 hours and will yield values lower than normal 24-hour values. Unless precautions are taken some birds will regurgitate food which will again be eaten at a time nearer the period of measurement; this will result in an increase in both the respiratory quotient and the metabolism. The effect of the fasting is increased if the birds are held at low temperatures, or if fighting or other marked activity is permitted, during the 24-hour fasting period. The fasting period must therefore be made a general preparatory period in which food, temperature and activity are fully equalized and controlled in a specially constructed cage. It is found that prolonged fasting results in a distinct decrease of the metabolism in both the pigeon and the dove. Measurements not made with the bird in approximate repose should, of course, be rejected. In addition, it is found that to avoid any “after effect” of activity or excitement of the bird during its transfer to the respiratory chamber the measurement should begin only one hour after the bird is placed in the chamber. Temperature is an important factor in determining the rate of heat production in doves and pigeons. Between 20° and 30° C. each degree modifies this rate approximately 2 per cent. Light must be excluded from the bird while in the respiratory chamber. These birds show a greater tendency to movement in the light than in the dark, during both day and night. In addition, it is quite possible that light here has a specific effect in increasing the metabolic rate. Measurements should be made exclusively at night, the normal period of repose for these animals. Tests made at 30° C. during the daytime, in darkened chambers into which light is passed only at intervals for temperature readings, yield values approximately 15 per cent higher than those obtained at night in continuously darkened chambers. Truly “basal” values from doves and pigeons are to be sought among measurements made at night in completely darkened chambers. The hours from 1:00–5:00 A.M. should probably be avoided, since especially low body temperatures and lowest blood pressures have been found to occur in these animals at this time. Our tests have indicated that measurements necessarily accompanied by fasting periods should not be soon repeated on the same bird. Most birds, though not all, will show a lower heat production in a second measurement which is made within 7 days of an earlier test. Prolonged inactivity and close confinement prior to measurement markedly decrease the metabolism. Doves confined in very small cages, for a period certainly as short as four weeks, do not then have a normal basal metabolism; their normal “basal” has been diminished. Immature but full-grown doves and pigeons have somewhat higher rates of heat production than birds at older ages; this is probably a true age effect. A study of the factors which modify the rate of heat production in doves and pigeons has demonstrated that such factors exist in very considerable number and complexity. It is thought that this investigation of these factors, together with the development of the apparatus and technique described in the preceding paper, now makes it possible to obtain reliable measures of the basal metabolism of these animals.