The present communication documents LH- and forskolin–-induced activation of adenylate cyclase (AC) system and progesterone synthesis in corpora lutea from pregnant ewes. The activation of AC in plasma membranes by LH or forskolin was amplified by Gpp(NH)p. These results suggest that regulatory nucleotide component (Ns) of the AC complex is required for full expression of the maximal response of ovine luteal AC to forskolin. Simultaneous addition of maximal concentrations of forskolin (10–4M), Gpp(NH)p (10–4M) and LH (10–7M) led to greater than additive (i.e. synergistic) responses: the experimental value was 4.71±0.19 nmoles cAMP/mg of membrane protein, whereas the theoretical additive effect was 3.17±0.10 nmoles/mg of membrane protein (p<0.001). These data reveal that more Nsor C component is being activated in these cells when combined treatments with these agents are applied.In intact cells maximum stimulatory concentrations of forskolin or LH caused similar increase in progesterone production with similar time courses. In striking contrast, the exposure of the luteal cells to LH and forskolin simultaneously led to a decrease in progesterone synthesis as early as 1h30 (40 %, p<0.001). Thus, the synergism observed between LH and forskolin on the stimulation of plasma membranes AC activity did not occur in steroidogenesis. The AC responses in crude plasma membranes form these cells to different stimulants were enhanced (i.e. 15%, p<0.2 for Gpp(NH)p, 33%, p<0.01 for LH plus Gpp(NH)p and 52%, p<0.01 for forskolin). These findings suggest that an early desensitization of the AC system cannot explain the impaired steroidogenic response observed.