Phospholipid-dependent, Ca2+-activated protein kinase (C-kinase) was recently shown to be expressed in rat pituitary. The enzyme is activated by Ca2+ and phosphatidylserine (PS). Diacylglycerol (DG), which is liberated during phosphoinositide turnover, and the potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA) activate pituitary C-kinase in the presence of PS, even at resting levels of intracellular Ca2+ (10(-7) M), and increase the apparent affinity of the enzyme for Ca2+. While micromolar concentration of Ca2+ had no effect on the apparent affinity of the enzyme for PS (Km approximately 15 micrograms/ml), elevation of Ca2+ to the millimolar range produced a sharp increase in the apparent affinity for PS (Km approximately 5 micrograms/ml). Elevation of PS (up to 500 micrograms/ml) could not replace Ca2+ in supporting maximal enzyme activity even in the presence of DG. Cytosolic pituitary C-kinase (70% of total enzyme activity) is recovered in an inactive state and can be activated without further purification. The particulate enzyme (30%) is recovered in a cofactors-insensitive form but can be activated after detergent-solubilization and anion exchange chromatography. Endogenous redistribution of soluble pituitary C-kinase to the membrane does not convert it to its proteolytic product which is insensitive to Ca2+, PS and DG. Pituitary C-kinase characterized here most likely plays a key role in signal transduction mechanisms involved in pituitary functions.