Suramin, a polyanionic compound which has been used in the treatment of trypanosomiasis and oncocerciasis, has recently been used in treatment of AIDS, while preliminary success has been reported in the treatment of cancer. However, suramin also causes adrenal insufficiency. The present study was undertaken in order to investigate the effect of suramin on ACTH-stimulated steroid production by dispersed rat adrenal cells. It was shown that suramin at concentrations of 10(-4)-10(-3) M inhibits ACTH-stimulated corticosterone release in a dose-dependent manner IC50 (2.10(-4) M). In addition, suramin caused a parallel decrease in ACTH-stimulated pregnenolone, progesterone and corticosterone release, suggesting that suramin does not affect corticosteroidogenesis via an inhibition of its regulatory enzymes. Suramin at 10(-4) M did not inhibit cholera toxin (10 mg/l)-, forskolin (5 microM)- and dbcAMP (5 mM)-stimulated corticosterone release, while cholera toxin completely overcame the inhibitory effects of very high concentrations of suramin (up till 10(-3) M), on ACTH-stimulated corticosterone release. Finally, chromatographic studies with a matrex gel showed that suramin directly interacted with the ACTH molecule. In conclusion, suramin at "therapeutic" concentrations (10(-4) M and higher) prevents ACTH-stimulated corticosterone release probably via a direct interaction with the ACTH molecule.