Urinary ammonium excretion, in vitro ammoniagenesis and the activities of renal cortical phosphate-dependent glutaminase (PDG) and glutamic dehydrogenase (GLDH) were measured in rats with a reduced renal mass. Following contralateral nephrectomy, ammonium excretion per nephron, ammonia production and the activities of PDG and GLDH were all increased significantly in remnant kidneys of rats fed high protein diets. In rats fed low protein diets, although PDG activity increased, GLDH activity and ammonia production and excretion did not increase in remnant kidneys following contralateral nephrectomy. Ammonia production and excretion were greater in rats fed high than low protein diets, a difference that was corrected by the addition of mineral acid to the diets of low protein-fed rats. Acid supplementation to the low protein group did not result in enhanced ammonia production or GLDH activity following a reduction in renal mass. The data indicate that the increased rate of ammoniagenesis which occurs following nephron reduction is markedly influenced by dietary protein content. A lack of enhanced GLDH activity may underlie the lack of increased ammonia production of low protein-fed rats following nephron reduction.