The effect of cycloheximide, an inhibitor of protein synthesis, on basal and stimulated renin secretion was examined in the isolated perfused rat kidney. Cycloheximide (0.05 mmol) was administered one hour before preparing the kidneys for perfusion. Both basal renin secretion and the response to intrarenal infusion of isoprenaline (0.06 nmol/min/g) or glucagon (0.1 nmol/min/g) were consistently reduced in the cycloheximide-treated group, suggesting dependence of these processes on protein synthesis