Nitric oxide (NO) is an important intra- and intercellular messenger controlling many biological processes. It is synthesized by NO synthases, which have been found also in granulosa cells. The present study examined whether NO is present in bovine follicular fluid and is produced by granulosa cells in culture. Secondly, it aimed to determine if NO affects the main parameters of granulosa cell function. The NO donor S-nitroso-L-acetyl-penicillamine (10(-3), 10(-4), 10(-5) M) was used to evaluate whether NO might influence steroidogenesis, proliferation and apoptosis in bovine granulosa cells collected from follicles divided according to their size in small (<5 mm) and large (>8 mm). The data demonstrate the presence of NO in follicular fluid and its production by granulosa cells in culture: the most active cells in producing NO are those from the small follicles, as confirmed by the NO levels in follicular fluid. This study also shows that NO donor significantly (P<0.001) inhibits progesterone (P4) and oestradiol 17beta (E2) production by the granulosa cells from both kinds of follicle; moreover, the highest concentration of NO donor significantly (P<0.001) inhibits DNA fragmentation in all the cells whereas the lowest concentration stimulates (P<0.001) cellular apoptosis only in granulosa cells from large follicles. NO donor does not seem to modify cell proliferation. Taken together these data lead point to NO as a local modulator of granulosa cell function.