Four distinct types of K+ channels were identified in rat and bovine adrenal zona glomerulosa (ZG) cells and characterized using single-channel recording techniques. Inward rectifier channels were the most frequently observed K+ channel types in the membrane patches of both rat and bovine ZG cells. The slope conductance of the inward current was 42 pS with an extracellular K+ concentration of 150 mM. The probability of the open state of these channels increased with depolarization. With the use of inside-out membrane patches with symmetric 150 mM K+ solutions, the rectifying behavior was found to require Mg2+ on the intracellular side of the membrane. Delayed rectifier K+ channels with conductances of 27 and 48 pS were found with rat ZG cells. These channels persisted with prolonged positive voltage steps and showed long mean open times with increasing depolarization. Transient outward currents with a conductance of 28 pS were observed only in bovine ZG cells. These channels showed substantial inactivation during positive voltage steps of 250 ms duration. Ca(2+)-activated K+ channels with a large conductance (228 pS) were identified in rat and bovine ZG cells. These different classes of K+ channels may be important for the control of resting membrane potential and the generation of action potentials, thus participating in the regulation of Ca2+ influx and aldosterone secretion in ZG cells.