The rapid effects of steroid hormones on Ca²⁺ signals have been examined in bovine adrenocortical cells. Among the steroid molecules tested, only corticosterone rapidly stimulated Ca²⁺ signals upon addition of ACTH, although corticosterone alone did not induce Ca²⁺ signals. Corticosterone also enhanced steroidogenesis induced by ACTH. The enhancement of ACTH-induced Ca²⁺ signals was also observed with membrane impermeable corticosterone conjugated to BSA, and was not inhibited by cycloheximide. In addition, corticosterone did not enhance Ca²⁺ signals induced by ATP or angiotensin II. These results suggest that corticosterone selectively stimulates ACTH-induced Ca²⁺ signals in a nongenomic way by acting on a target in the plasma membrane. Furthermore, the supernatants of cells incubated with ACTH or ATP enhanced Ca²⁺ signals--suggesting that steroids produced by such treatment act in an autocrine fashion. Consistent with this idea, these effects were inhibited by inhibitors of steroidogenesis (aminoglutethimide or metyrapone). These results show that steroid molecules synthesized in adrenocortical cells facilitate ACTH-induced Ca²⁺ signals. Taken together, corticosterone secreted from adrenocortical cells activates ACTH-induced Ca²⁺ signals and steroidogenesis by nongenomic means.