Our current working hypothesis for the intracellular mechanism of action for LH and PGF-2 alpha is shown in Fig. 8. Luteinizing hormone appears to act primarily on the small luteal cell through the cAMP/protein kinase A effector system and thereby stimulates secretion of progesterone. Activation of the protein kinase C effector pathway is inhibitory to progesterone secretion from stimulated small luteal cells but it is not clear which hormones, if any, activate this effector system. Results from studies in whole animals suggest that LH may also stimulate differentiation of small luteal cells into large luteal cells (Donaldson & Hansel, 1965; Farin et al., 1988). Although there are LH receptors on large luteal cells, LH treatment does not stimulate progesterone secretion and does not appear to activate any of the second messenger pathways which we have examined. Prostaglandin F-2 alpha appears to act on the large luteal cell through free intracellular calcium and protein kinase C effector systems. Apparently, PGF-2 alpha-induced activation of protein kinase C results in the acute inhibition of progesterone production seen in the first 8 h after PGF-2 alpha treatment. The cytotoxic effects of PGF-2 alpha on the large luteal cell (Fitz et al., 1984; Braden et al., 1988) may be caused by a sustained elevation in free intracellular calcium concentrations. No direct effects of PGF-2 alpha on small luteal cells have been detected (no inhibition of progesterone production, no activation of protein kinase C, no increase in free intracellular calcium), which is consistent with an absence of high affinity PGF-2 alpha receptors on this cell type. The cytotoxic effects of PGF-2 alpha on small luteal cells and endothelial cells (Braden et al., 1988) may be caused by decreases in luteal blood flow (Niswender et al., 1975; Wiltbank et al., 1990b), actions of cytotoxic agents released by large luteal cells, or increases in cytotoxic white blood cells (Murdoch, 1987; Bagavandoss et al., 1988).