Sheep somatotroph-enriched cultures were obtained by means of enzyme dissociation and Percoll gradient separation. Nystatin-perforated-whole-cell recordings were performed on post-recording-identified somatotrophs after 4–14 days in vitro. Using Ca2+-free, tetrodotoxin-containing (1 µM) bath solution and K+ electrode solution, three types of voltage-dependent K+ currents were recorded as inward rectifying, outward transient and outward delayed rectifying K+ currents. The inward rectifying K+ current was very small at physiological extracellular K+ concentrations (5 mM) and enhanced by increasing the K+ concentration in the bath to 55 mM; it was blocked by tetraethylammonium (2 mM) but not by 4-aminopyridine (5 mM). A transient outward K+ current appeared at –50 mV and was selectively diminished by 4-aminopyridine (2 or 4 mM). A delayed rectifying outward K+ current was observed when the membrane potential was depolarized to -20 mV and was blocked by tetraethylammonium (2 mM) but not 4-aminopyridine (4 mM). Application of 4-aminopyridine but not tetraethylammonium (up to 5 mM) depolarized the cell membrane potential recorded under current clamp conditions and triggered action potentials when the bath solution contained Ca2+ (2 mM) but not tetrodotoxin. The intracellular Ca2+ concentration was increased by 4-aminopyridine as was growth hormone release. Therefore, the 4-aminopyridine-sensitive transient outward K+ current appears to be important in the determining the resting potential of ovine somatotrophs and plays a major role in regulating basal intracellular Ca2+ concentration and growth hormone secre