Substantial synthesis of gamma-aminobutyric acid occurs in rat renal cortex. Renal glutamate decarboxylase activity (24.3 +/- 2.9 (S.E.) nmols/mg protein per h) is 15% of that in brain; renal gamma-aminobutyric acid content (39.5 +/- 5.3 (S.E.) nmols/g wet wt.) is 5% of the whole brain concentration. Properties of glutamate decarboxylase were studied in homogenates of rat renal cortex and rat brain under conditions for which gamma-aminobutyric acid formation from [2,3-3H]glutamate and CO2 release from [1(-14)C]glutamate were equal. Several properties of renal glutamate decarboxylase distinguish it from the corresponding brain enzyme: (1) renal glutamate decarboxylase is selectively inhibited by cysteine sulfinic acid (Ki = 5X10(-5) M); (2) renal glutamate decarboxylase is less sensitive (Ki = 3-5X10(-5) M) to inhibition by aminooxyacetic acid than is the brain enzyme (Ki = 1X10(-6) M); (3) brain but not renal glutamate decarboxylase activity can be substantially stimulated in vitro by the addition of exogenous pyridoxal 5'-phosphate; (4) renal glutamate decarboxylase is significantly decreased in renal cortex from rats on a low-salt diet. Proximal tubules are enriched in glutamate decarboxylase compared to the activity in whole renal cortex or glomeruli (42, 22 and 14 nmols/mg protein per h, respectively). We speculate that renal gamma-aminobutyric acid synthesis does not reflect the presence of GABAergic renal nerves, but may serve a function in proximal tubular cells.