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1. Following a single intravenous injection of inulin, values for the renal clearance of inulin (Cin = µν/P) show a steady decline, with diminishing values of plasma inulin concentrations. 2. Employing values of P 6 minutes before the mid‐point of the clearance period, in place of values 2½‐minutes before the mid‐point, does not eliminate this phenomenon. 3. Employing the formula Cin = µν/P, the fall in values for clearance is due to the fact that the renal excretion of inulin (µν) is not proportional to the plasma level (P), since the imposition of a linear relationship between the two variables results in lines which deviate significantly from the point of origin. 4. The lack of proportionality is shown not to be due to the sampling of venous blood as an estimate of arterial plasma inulin concentrations. 5. The dependence of the magnitude of the renal clearance of inulin upon the plasma concentration of inulin is found also in three subjects using the continuous infusion method of administration. This excludes the possibility that the results obtained, following a single injection of inulin, are distortions of a simple proportional relationship consequent upon the dynamics of distribution of inulin in the body following a single injection. 6. The results of the experiments, and those of other workers, are held to suggest strongly that, in man, inulin is reabsorbed by the tubules to a significant extent. Hence the use of inulin clearance as a measure of glomerular filtration rate involves a systematic error, which is estimated to be about 15 per cent. under the usual circumstances.

The plasma volume was measured by T1824 dilution in groups of normal young male subjects at regular intervals over the course of 3 years. The plasma volume increased during the summer and decreased during the winter, but this change was variable from subject to subject and was not as marked during 1 year as during the other 2. Ascorbic acid in daily supplement of 400 mg., taken during the first year of the study, did not influence the plasma volume fluctuations. The red blood cell mass, circulating haemoglobin, and total circulating plasma protein varied in the same way and the mean corpuscular haemoglobin concentration also increased during the summer in 2 of the 3 years. The changes in plasma volume are attributed to the influence of environmental temperature and indicate some degree of seasonal acclimatization. Serial measurements of total body water and extracellular space on a few subjects suggest that the seasonal fluctuation affects mainly the vascular system.

Parotid saliva was collected into graduated tubes by fitting small cups over the parotid papillae. The sodium, chloride, and bicarbonate concentrations and the pH of parotid saliva increased with increasing rates of secretion, but the potassium and phosphate concentrations remained constant at secretion rates greater than 0.5 ml. per minute. Although the latter two ions were always at concentrations higher than serum levels, the parotid juice was always hypotonic but decreasingly so as the rate of flow increased. There was appreciable variation between individuals and also in the same subject from time to time. No difference was noted between the two gustatory stimuli used, dilute acetic acid and glucose candy. The potassium concentration in parotid saliva was higher than in mixed submaxillary and sublingual saliva. Continuous recording of the rate of secretion on a smoked drum showed cyclic variations from minute to minute, indicating that timed collections over a period of minutes give mean rather than actual rates of flow.

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