Cyclosporin A is known to produce increases in plasma and kidney renin in vivo. In this study, Cyclosporin A was shown to stimulate renin release in vitro in rat renal cortical slices. At the optimal concentration of 8 x 10(-6) M Cyclosporin A, renin release was stimulated by up to 43% during a 2 hr incubation at 37 degrees. This observation strengthens the hypothesis that the intra renal renin-angiotensin system may participate in the mechanism of Cyclosporin A nephrotoxicity.