To evaluate the role of each nephron segment in renal ammoniagenesis, distribution of renal ammoniagenic activity along the nephron in control and acidotic rats was examined. We used our original aerobic incubation system and ammonia produced from glutamine in 7 defined segments of microdissected nephron was measured using the enzymatic cycling method. When ammonia production in the control was compared in each nephron segment, the highest specific activity of ammoniagenesis per mm tubular length and that per microgram protein were observed in the proximal straight tubule (PST) and the thick ascending limb of Henle's loop, respectively. Chronic metabolic acidosis increased ammonia production per mm tubular length markedly in the proximal convoluted tubule (PCT) (+171%), moderately in the medullary collecting tubule (+123%) and PST (+77%), and slightly in the distal convoluted tubule (+52%), revealing that the highest activity of ammoniagenesis was located in PCT and PST in acidosis. These data indicate that proximal tubules have major roles in renal ammoniagenesis both in the control and in acidosis. From the early observation of glutaminase I isoenzyme distribution along the nephron, our data suggest that not only phosphate-dependent glutaminase but also phosphate-independent glutaminase may have important roles in renal ammoniagenesis.