Phosphate- independent glutaminase (PIG) and gamma-glutamyl transpeptidase (gamma GT) activities were evaluated in renal cortex homogenates of rats under normal acid-base balance or chronic metabolic acidosis (CMA). An incubation medium containing glutamine 2 mM, no phosphate, pH 7,40, was used. PIG activity was measured as glutamate production and total gamma GT activity as ammonia production. In normal rats gamma GT activity was significantly higher (0,84 +/- 0,05 mumol/min/g wet wt.) than PIG activity (0,48 +/- 0,06 mumol/min/g wet wt.) (p less than 0,01). In CMA there was a significant increase in PIG activity and an even higher increase in gamma GT activity (p less than 0,01 compared to controls in both cases). The glutamate production/ammonia production ratio was 0,57 +/- 0,05 in normal rats, and 0,37 +/- 0,03 in CMA ( p less than 0,025). These data suggest that the increase in PIG activity and, to a further extent, the increase in gamma GT activity may play an important role in augmenting renal ammonia production in CMA.