These studies utilized a prolactin-secreting clonal strain (2B8) of rat anterior pituitary cells. These cells secrete increasing amounts of prolactin in response to thyrotrophin releasing hormone (TRH) and do not produce any other pituitary hormone. In response to TRH and in the presence of calcium in the culture medium the cells demonstrated evoked responses without spike discharge following electrical stimulation. No spontaneous evoked response with spike discharge was observed in 2B8 cells. The evoked responses were not suppressed by tetrodotoxin (TTX) but were markedly suppressed by D-600, or lanthanum chloride. The results suggest that calcium ion flux across the cell membrane may be a necessary component of the intracellular events which lead to the release of prolactin.