Aqueous solutions of 'ammonia' contain both the free base NH3 and its conjugate acid, the ammonium ion NH+, in proportions determined by the pH. The total of both species present in a solution may be referred to as 'ammonia'; it is usually estimated by removing the free base by diffusion, aeration or distillation in steam after the solution has been made alkaline. Since common urinary solutes do not modify the acidic dissociation constant pKa of ammonium (Bank & Schwartz, 1960), and since the pH of human urine is always well below this, 'ammonia' in urine is present almost entirely in the form ofammonium ions, which represent an important part of the acid excreted by the kidneys (Robinson, 1961).It is often stated that the rate of excretion of 'ammonia' is determined principally by urinary pH, in that it increases as the urine becomes more strongly acid. The influence of acidity was emphasized in a graph published by Eggleton (1947) showing a linear relation between the output of 'ammonia' and the pH of human urine in 148 observations on one subject. In contrast to this well-recognized effect of acidity, the rate of flow of urine has been supposed to exert little influence upon the excretion of' ammonia ', but Eggleton (1947) pointed out that the linear relation was found in only 13 of 30 subjects tested on at least two occasions; the output of 'ammonia' by the remaining subjects varied with the rate of flow to an extent which differed from one to another. An earlier study (Eggleton, 1946) of diuresis induced by alcohol had also indicated that any relation between 'ammonia ' output and urinary acidity was considerably modified by the rate of urine flow, for diuresis was associated with an appreciable increase in the excretion of 'ammonia', and such an increase might occur in spite of a decrease in acidity. Dependence of the rate of excretion of 'ammonia' upon urine flow has been well established in animals other than man. Richterich, Goldstein & Dearborn (1958) demonstrated a linear relation between minute volume and the output of urinary 'ammonia' in guineapigs and rabbits. These animals are herbivora, whose urine is usually