1. Whole-cell voltage-activated currents from single zona fasciculata (ZF) cells from rat adrenal glands were studied. T- and L-type Ca2+ currents and a slowly inactivating A-type K+ current were the three major currents observed. 2. In freshly isolated cells, the A-type K+ current and the T-type Ca2+ current were predominant. The A-type current was activated at -50 mV and inhibited by 4-amino-pyridine with a half-maximal block (IC50) at 130 microM while the T-type current was activated at -70 mV and blocked by Cd2+, Ni2+ and amiloride with IC50 values of 24.1, 132.4 and 518.9 microM, respectively. 3. Under current clamp, depolarizing current pulses produced a single Ca2+ action potential with Cs+ in the pipette internal solution. Upon replacement of Cs+ by K+, the half-amplitude width of the action potential was shortened and membrane potential oscillations were seen after the spike. 4. In freshly isolated cells and during the first 24 h after plating, the T-type current was observed in all cells, with L-type current being observed in < 2% of cells, even in the presence of (+)SDZ 202,791, a dihydropyridine Ca2+ channel agonist. With time in culture, the T-type current disappeared, and a high-voltage-activated L-type current became increasingly apparent. In cells tested after > 2 days in culture, (+)SDZ 202,791 potentiated L-type current by 407 +/- 12% and the antagonist (-)SDZ 202,791 blocked this increase. The L-type current was activated between -30 and -20 mV and was sensitive to nitrendipine and omega-conotoxin GVIA. 5. Pre-incubation of cultured ZF cells with adrenocorticotrophic hormone (ACTH) or vasoactive intestinal peptide (VIP) for 3 days resulted in a high, sustained level of expression of T-type current, with a mean amplitude of 34.2 +/- 5.5 pA pF-1 for ACTH-treated cells compared with 3.4 +/- 1.8 pA pF-1 for untreated cells. Cycloheximide strongly inhibited this effect. Neither treatment affected L-type current expression. 6. The expression of both Ca2+ current types was unaffected by pre-incubation with 8-bromo-cAMP or forskolin. The protein kinase A antagonist, H89, did not inhibit the ACTH-induced upregulation of T-type Ca2+ currents. 7. It is concluded that the main voltage-dependent currents involved in cell excitability and steroidogenesis in rat adrenal ZF cells are an A-type K+ current and a T-type Ca2+ current. The physiological role and control of expression of L-type Ca2+ channels in rat ZF cells remain less clear.