Previous investigations have established that spironolactone (SL) is converted to a reactive metabolite by adrenocortical NADPH-dependent enzymes, resulting in the destruction of microsomal cytochrome(s) P-450 and decreases in steroid hydroxylase activities. Hepatic microsomes, by contrast, do not activate SL. Studies were done to characterize the activation pathway by comparing adrenal with hepatic metabolism of SL in guinea pigs. In the absence of NADPH, both adrenal and hepatic microsomal preparations converted SL to its deacetylated metabolite, 7 alpha-thio-SL. NADPH had no effect on hepatic SL metabolism but stimulated adrenal metabolism of SL. In the presence of NADPH, very little 7 alpha-thio-SL was recovered from the adrenal incubations, suggesting that the 7 alpha-thio-SL was further metabolized by NADPH-dependent enzymes. The latter hypothesis was confirmed by incubating microsomal preparations with 7 alpha-thio-SL as the substrate. In the presence of NADPH, 7 alpha-thio-SL was rapidly metabolized by adrenal microsomes but was not metabolized by hepatic preparations. Under the same incubation conditions, 7 alpha-thio-SL promoted the destruction of adrenal cytochrome(s) P-450 but had no effect on hepatic monooxygenases. 7 alpha-Thio-SL was far more potent than SL in promoting the destruction of cytochrome(s) P-450, suggesting that the metabolite might be an intermediate in the actions of the parent compound. Indeed, inhibition of SL conversion to 7 alpha-thio-SL by the esterase inhibitor, diethyl p-nitrophenyl phosphate blocked the effects of SL on adrenal cytochrome(s) P-450. Diethyl p-nitrophenyl phosphate did not affect the actions of 7 alpha-thio-SL on cytochrome(s) P-450.(ABSTRACT TRUNCATED AT 250 WORDS)