Membrane electrical properties of the clonal anterior pituitary cell (GH₃) were studied using intracellular recording techniques. The resting potential in the GH₃cell was -48.0±1.1 mV (mean±S.E., n=39) in normal saline. The input resistance estimated from the linear portion of the current-voltage relationship was 453±30 MΩ(n=21). The maximum rate of rise of the action potential was 7.0±1.1 V/sec (n=12) at room temperature (23-25°C). The action potential had both Na and Ca components. The Ca component was abolished by addition of 4 mm Co²⁺. Sr²⁺ could substitute for Ca²⁺in supporting spike initiation. As the concentration of Sa²⁺ was increased, the maximum rate of rise of the action potential increased. After replacement of Ca²⁺with isomolar Ba2+ the membrane potential shifted to -6.1±1.1 mV (n=11). In the Ba solution, prolonged action potentials were evoked by a depolarizing current pulse after maintaining the membrane potential more negative than -50 mV. The release of both prolactin and growth hormone was enhanced by increasing the external K+ concentration to 50 mm in the presence of Ca²⁺. Sr²⁺could substitute for Ca²⁺. Ba²⁺enhanced the release of both hormones, even if the K+ concentration was unaltered. The facilitatory effects of high K⁺ and Ba²⁺were markedly suppressed by addition of 2-4 mm Co²⁺. These results may suggest that the potential-dependent increase in the membrane permeability to Ca²⁺, responsible for initiation of the Ca spike, plays a significant role in stimulation of hormone secretion in GH3 cells.