Abstract The secretion of ammonia, once thought to be limited to distal tubules and collecting ducts, has been shown to occur throughout the length of the nephron of the rat. As much as half or more of the ammonia excreted in the urine may be added to tubular fluid in the proximal segment. Additional evidence has been adduced that the passive diffusion of free-base ammonia (NH 3 ) from its site of production in tubular cells into acid tubular urine is a major mechanism of ammonia secretion. Passive diffusion from cells into peritubular blood adequately accounts for the observed rate of addition of ammonia to renal venous blood. Thus ammonia has been shown to diffuse rapidly and readily in both directions across the renal tubular epithelium of the dog and man along a gradient of hydrogen ion concentration. Ammonia in cells of the renal cortex is in diffusion equilibrium with blood in peritubular capillaries and probably with fluid in proximal and distal convoluted tubules. The partial pressure of ammonia in renal venous blood is a fair approximation of that of renal cortical cells. Although this evidence strongly supports passive nonionic diffusion, it does not exclude the possibility that a fraction of the ammonia may be secreted by the active exchange of ammonium ions for sodium ions. In both the dog and man, glutamine is the major precursor of urinary ammonia. Since the kidney extracts glutamine from renal arterial blood, yet excretes no glutamic acid in the urine and adds little to renal venous blood, it is evident that the amino as well as the amide nitrogens of glutamine must be potential sources of ammonia. Studies with glutamine labeled with N 15 in the amide position show that 35 to 40 per cent of the nitrogen of the urinary ammonia excreted by the acidotic dog is derived from amide groups. The amino nitrogen of glutamine and of other amino acids accounts for the remainder. The glutamine pool of the kidney is small and turns over rapidly, with a half-time of less than 2.5 minutes. Many factors are involved in the control of rate of excretion of ammonia, including plasma substrate concentration, activity of the mechanisms of transport of substrate into tubular cells, activity of enzymatic production of ammonia within cells, activity of mechanisms concerned with acidification of the urine and rate of urinary flow.