The gonadotropin-releasing hormone associated peptide reduces calcium entry in prolactin-secreting cells. The precursor molecule to the GnRH contains a peptide named GnRH-associated peptide (GAP) with PRL-inhibiting properties. In this work, we have studied the electrophysiological properties and responses to GAP of three different types of PRL-secreting cells: 1) the rat tumor cell line GH3, 2) normal rat pituitary cells in primary culture, and 3) human PRL-secreting adenoma cells. Using different but complementary techniques we show that GAP reduces intracellular Ca++ levels, [Ca++]i, and inhibits Ca++ transients in these cells. This reduction of [Ca++]i results from coordinate actions of GAP on K+ and Ca++ conductances and may explain the inhibitory effect of GAP on hormonal secretion by PRL-secreting cells. 