Normal rat pituitary cells were isolated and enriched to obtain a relatively pure population of growth hormone (GH) secreting cells by sedimentation at unit gravity and subsequently grown in culture. The electrophysiological properties of these cells were studied with intracellular recording techniques. 4-6-day-old cells did not show active electrical properties. At 7-9 days the cells were able to generate evoked responses (fast responses) which were blocked by inhibitors of calcium flux and prolonged with tetraethylammonium. The resting potential of these cells was sensitive to external local application of substances known to act on the release of GH. External elevated K+ induced a strong depolarizing response which was associated with an increase of membrane conductance. Somatostatin induced a hyperpolarizing response associated with a decrease of membrane conductance. It is suggested that the fast Ca2+-dependent responses and the slow variations of the resting potential induced by substances which modify hormone release may play an important role in the control of the secretory process of the presumably GH-secreting cells.