1. Voltage-dependent K+ currents were recorded in cultured tumour-derived anterior pituitary cells of the rat (GH3 cells) with the patch clamp technique. An inward-rectifying current is described which is found to be carried by K+. 2. In isotonic KCl, whole-cell inward K+ currents were elicited by hyperpolarizing pulses from a holding potential of -40 mV. These inward K+ currents showed time- and voltage-dependent inactivation at potentials more negative than -60 mV. Inactivation was faster and more complete at larger hyperpolarizations. Recovery from inactivation was also time- and voltage-dependent. It was faster and more complete with more positive potentials. Time course of inactivation and of recovery from inactivation could be fitted by single exponentials. 3. Two results showed that a steady inward K+ current is present at -40 mV. The holding current at -40 mV was reduced following complete inactivation of the inward K+ current during strong hyperpolarizing pulses, and the amplitude of maximum inward K+ current elicited by hyperpolarization increased after depolarizing pre-pulses of 5 s. The resting conductance was estimated to be 20-30% of the maximum inward-rectifying conductance. 4. The inward K+ current was drastically reduced by Cs+ and Ba2+, but not by Ni2+ and Co2+. Quinidine, 4-aminopyridine and tetraethylammonium chloride blocked the current. In contrast, dendrotoxin was without effect. 5. Thyrotrophin-releasing hormone (TRH) which induces biphasic secretion of prolactin in GH3 cells consistently reduced the inward K+ current in the presence of internal Ca2+. This reduction was abolished if the pipette solution contained guanosine 5'-O-(2-thiodiphosphate) (GDP beta S; 400 microM), confirming the involvement of G-proteins in the signal transduction pathway. 6. TRH shifted the voltage-dependence of inward K+ current inactivation to less negative potentials resulting in pronounced K+ current inactivation in the range of the resting potential of these cells (-40 to -60 mV). 7. In intact cells, closing of K+ channels would result in a depolarization. The existence of an inward-rectifying K+ current in GH3 cells which is able to be reduced by TRH could readily explain the TRH-induced increase in action potential firing underlying the sustained second phase of secretion.