We examined the presence and the regulation of nitric oxide (NO) synthase (NOS) using in vitro cultured corpora lutea (CL) obtained from rabbits at days 4 and 9 of pseudopregnancy. The role of NO and NOS on steroidogenesis was also investigated using the same CL preparations after short-term incubations (30 min and 2 h) with the NO donor, sodium nitroprusside (NP), the NOS inhibitor, Nomega-nitro-l-arginine methyl ester (l-NAME) and prostaglandin (PG) F-2alpha. The basal NOS activity was greater in CL at day 4 than at day 9, and was also differently modulated by PGF-2alpha, depending on the age of the CL. The addition of PGF-2alpha to day 4 CL had no effect, but PGF-2alpha on day 9 caused a threefold increase in NOS activity. NP caused a two- to fivefold decrease in release of progesterone from CL of both ages, and this inhibitory effect on steroidogenesis was reversed by l-NAME. All treatments failed to modify basal androgens and 17beta-oestradiol was not detectable in either control or treated CL. These results suggest that NO is effectively involved in the regulation process of steroidogenesis, independently of 17beta-oestradiol. PGF-2alpha had no effect on day 4, but induced luteolysis on day 9, by reducing progesterone (P

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