The effect of RU 28318, a specific and highly potent aldosterone antagonist on mineralocorticoid biosynthesis has been studied using a new in vitro model which combined three original features: (1) a very specific radioimmunoassay for aldosterone (2) a simplified perifusion system and (3) frog interrenal tissue which spontaneously produces high amounts of aldosterone. A dose-related inhibition of aldosterone production was observed for doses ranging from 10(-5) to 10(-3) M of RU 28318. The intermediate dose of 10(-4) M caused 71% inhibition of aldosterone production. Long term infusion of RU 28318 for 8 h led to a significant, stable and reversible inhibition of aldosterone production. In addition, we provide evidence that RU 28318 is capable of blocking the stimulation of aldosterone secretion induced by synthetic ACTH or by angiotensin II analogue. The present results demonstrate that RU 28318 is responsible for a significant and reversible inhibition of spontaneous, ACTH-induced and angiotensin II-induced aldosterone biosynthesis in vitro.