III. This paper is the 3rd installment of a series describing the physiological changes observed in 2 men and 3 women during the 2-yr. period 1925-1927. Statistical measures are given of: the intra-individual variability of these functions; the degree of correlation between the components of the alveolar air and the barometric pressure; and the correlation between the alveolar CO2 tension and the blood CO2 capacity. The striking effect of menstruation on the alveolar CO2 percentage and tension is the most definite effect of menstruation that the authors have observed in all of the functions of which they have kept record. The alveolar air shows a seasonal variation in composition which the authors believe is related, the O directly and the CO2 inversely, to the av. barometric pressure. The blood gas capacity also shows a seasonal variation; the CO2 capacity is quite conclusively lowest during the summer; this is also true of the O capacity for 1926 and reasons are given for believing that this is probably the typical effect. In addition, the blood gas capacity shows a progressive increase throughout the 2 yrs. which the authors were unable to account for except, perhaps, as a stimulating effect on blood regeneration of taking the weekly blood samples.[long dash]IV. For each of the functions studied (variations in vital capacity, rate and volume of the respiration and composition of the expired air) are given the statistical constants denning the modes and means and the extent and degree of variability. In addition the authors calculated the degree of correlation between the rate and minute and tidal volumes of the respiration; and between these and the O consumption and CO2 production. There is an unusually high correlation between the volume of pulmonary ventilation and the CO2 output; reasons are given for believing that this is not due to increased breathing washing out increased amts. of CO2, but is an expression of the fundamental control of pulmonary ventilation by the rate of CO2 production. The results of the 2 yrs. are sharply separated as regards the minute and tidal volumes and the composition of the expired air; this was due to the dead space in the apparatus for collecting the expired air being large (about 50 cc.) during the 1st yr. and small or practically negligible during the 2nd. The large dead space was associated not only with large absolute values for these functions, but occasioned, also, a much greater degree of variability. Going to sleep (dozing) during the collection of the expired air decreases the minute volume 3-4%; it is not clear to what extent this is due to variations of rate or of tidal volume. Menstruation had no effect on vital capacity or the rate of respiration; on the other hand, the volume of pulmonary ventilation and the composition of the expired air are definitely affected. It is shown that the menstrual variations in metabolism, pulse rate, composition of the alveolar air and blood gas capacity described in previous reports may be consistently correlated with the changes described here. Neither the vital capacity nor the respiratory rate can be seen to have a seasonal variation; the tidal and minute volumes and the composition of the expired air, however, show the most definite and uniform variation of any of the functions studied except the basal pulse rate.