1. Voltage-dependent K+ currents were studied in normal rat anterior pituitary cells using the patch-clamp technique. To obtain cultures enriched for lactotrophs, density gradient centrifugation was performed on pituitary cells isolated from lactating rats. 2. Depolarizations to about -30 mV from a holding potential of -80 mV activate a rapidly inactivating [time constant (τ) ~ 15-20 ms at -20 mV] K+ current. This transient current activated at low voltages (termed I(A)) is abolished by 5 mM external 4-aminopyridine (4- AP) but is largely resistant to external tetraethylammonium (TEA) (≤30 mM). 3. Recovery from inactivation of I(A) is fast, with a τ of 100-200 ms at - 80 mV. Deactivation is also fast (τ ~ 2.2 ms at -50 mV). The voltage of half-activation of I(A) is approximately -20 mV. The current is completely inactivated at a holding potential of -40 mV. 4. Voltage-dependent K+ current activated by depolarizations from a holding potential of -40 mV was first detectable at about -20 mV (high voltage activated) and had a time course that varied among cells. 5. Deactivation of high voltage-activated K+ current was best described by the sum of two exponentials, with τ of about 3.7 and 30 ms at -50 mV. Both components reversed close to the equilibrium potential for K+. 6. The amplitudes of the two tail currents were independent of each other when variable-duration commands were used to activate current. The amplitude of the fast component was largest with 10- to 20-ms commands to +40 mV and was reduced (≤50%) with 136-ms commands. The slow component amplitude reached a peak by 40 ms and remained constant for commands ≤136 ms at 140 mV. 7. The contribution of each component to the total high voltage-activated tail current was variable among cells, with the amount of fast component correlating with the amount of inactivation produced by commands to +40 mV. 8. The two components of tail current activated by depolarizations from the -40 mV holding potential were abolished by external TEA (10 mM). 4-AP (5 mM externally) selectively abolished the fast component of high voltage-activated tail current while only partially reducing the slow component. 9. These results suggest that normal rat anterior pituitary cells possess at least three distinct types of voltage-dependent K+ current; a low voltage-activated, transient current (I(A)) and two high voltage-activated currents. The differential expression and/or modulation of these currents may be important in shaping the electrophysiological response of pituitary cells to secretagogues.