The role of parathyroid hormone (PTH) in ammonium metabolism in the rat remnant kidney was studied by examining the effects of parathyroidectomy (PTx) in rats with intact kidneys and with 5/6 nephrectomy (Nx). PTx in rats with intact kidneys caused a rise in urine pH and a decrease in urinary ammonium excretion without affecting in vitro ammonium production rate or the ammonium content in the cortex. Unexpectedly, the ammonium content in the medulla was markedly reduced by PTx so that the corticomedullary ammonium gradient was inverted. As compared to control rats, rats with 5/6 Nx had a lower urinary ammonium excretion rate, a higher in vitro ammonium production rate, and an increase in ammonium content in both cortex and medulla with reduced corticomedullary ammonium gradient. PTx in rats with 5/6 Nx led to a further decrease in urinary ammonium excretion, attenuated the increase in the in vitro ammonium production rate, and lowered the ammonium content in both cortex and medulla with inverted corticomedullary ammonium gradient. These effects of PTx in Nx rats were corrected by continuous PTH infusion with Alzet minipump. In summary, results from these studies indicate that PTH plays an important role in maintaining the urinary ammonium excretion. In rats with intact kidneys, PTH contributes to urinary ammonium excretion by increasing urinary acidification and medullary ammonium accumulation. In rats with reduced nephron mass, PTH enhances urinary ammonium excretion by stimulating ammonium production and retaining medullary ammonium in the remnant kidney.