The effect of Cyclosporin A (CyA) on renin storage and release in rats was investigated. Renin release from incubated renal cortical slices was increased in rats treated with 25 or 50 mg/kg/day of CyA for 3 to 7 days. This was associated with an increase in renal cortical renin content, which was also increased over the same time period. Plasma renin activity was similarly elevated in these rats. It is possible that these increases in renin are related to the nephrotoxicity of CyA.