Gonadotrophs of the ovine pars tuberalis have been studied using the patch clamp technique for recording of single ion channel currents. We report that gonadotrophin-releasing hormone (GnRH) acts on these cells to open an inward-current cation channel which is permeable to Ca2+. When measured in cell-attached patches with 5 mM extracellular Ca2+, the GnRH-activated channel has a unit slope conductance of 8.0 ± 2.6 pS (range 4–14 pS). The channel conductance is increased to 13.6 ± 2.4 pS when the external medium contains 95 mM Ba2+ as the charge carrier. GnRH action appears to be mediated through an internal messenger system, since GnRH does not need to be in direct contact with these channels in order to cause their opening. This internal messenger system is unlikely to be Ca2+ itself. In addition, two other voltage-dependent outward-current channels have also been detected, both of which are permeable to K+, but differentiated in the cell-attached recording mode by widely different conductance values of 20–30 and 100–120 pS, respectively, and a reversal potential of –90 to –100 mV. The higher conductance channel is sensitive to internal Ca2+, and its probability of opening is increased in the prese