Progesterone (P₄) production by the bovine placenta differs from that of other steroidogenic tissues in two important aspects: 1) it is calcium dependent but cyclic nucleotide-independent and 2) it is suppressed by an endogenous inhibitor for most of the life span of the placenta. This natural refractory state of the placenta can be overcome by in vitro incubations of fetal cotyledon cells by agents that increase intracellular calcium [isobutyl methyl xanthine (MIX); calcium ionophore (A23187)], stimulators of protein kinase C (PKC), such as phorbol ester (TPA) and addition of substrate (pregnenolone, 25-OH hydroxycholesterol). Staurosporine (STA), which is known to be an inhibitor of PKC, PKA and tyrosine kinase (TK), was found to stimulate steroidogenesis rather than to inhibit. However genistein, TK inhibitor, did inhibit P₄ production in a dose dependent manner.  It was also found that both MIX and A23187 stimulate both transcription (mRNA) and translation of the side chain cleavage (SCC) enzyme as determine by RT-PCR and Western blot analysis respectively. In contrast Staurosporine stimulation of P₄ was not associated with increased level of mRNA or SCC protein, while genistein inhibition of P₄ was associated with inhibition of transcription and translation of SCC. Data suggest that STA could act either to indirectly inhibit kinases involved in the regulation of the inhibitor or 2) act directly on the phosphorylation of the steroidogenic enzyme. However the possibility that STA mediate a novel protein kinase that has not been defined can not be excluded.