The influence of posture on output of water, bicarbonate, chloride, inorganic phosphate, inorganic sulphate, urea, ammonia, creatinine and titratable acid, on urine pH, on blood pressure, pulse rate and circulation rate was studied. Observations were made for 18 2-hour periods in the recumbent posture and 18 standing, on 3 subjects. In every case circulation rate and pulse pressure were greater in the recumbent than in the corresponding standing periods, and in all but 1 subject the mean arterial pressure was lower in the recumbent. In every case the output of water, bicarbonate and chloride was greater in the recumbent position (average increases for all subjects in all experiments of recumbent output over standing are: water, 237%; CO2, 549%; NaCl, 123%); the output of phosphate, of sulphate and of urea was usually somewhat increased in the recumbent (P, 50%; S, 41%; urea N, 64%); the output of ammonia and of creatinine was not consistently affected (average ammonia N, [long dash]8%; creatinine, 10%). The data are believed to justify the following conclusions: (1) the number of glomerular capillaries exhibiting an active circulation is greater in the recumbent than in the standing posture; (2) rate of filtration by the individual glomerular units is increased in the recumbent posture; (3) rate of filtration across a glomerular membrane is determined not only by the mean glomerular capillary pressure but also by the extent to which the application of pressure is pulsatile; (4) urea, sulphate and phosphate are partly filtered through the glomeruli and partly secreted by the tubules; (5) ammonia and creatinine are eliminated solely, or almost solely, by tubular secretion.