In the present study, we have examined the possible involvement of the central nitric oxide (NO) pathway in the control of prolactin secretion in vivo. The effects of intracerebroventricular (i.c.v.) injections of L-arginine (L-Arg), a precursor of NO, N omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of NO synthase (NOS), and of sodium nitroprusside (SNP) and 3-morpholino-sydnonimine (SIN-1), NO donors, on basal prolactin levels were studied in conscious male rats. Microinjections of L-Arg (100 and 500 mu g) or L-NAME (100 and 500 mu g) did not modify plasma prolactin levels, however i.c.v. injections of both SNP (1, 5, 10 and 20 mu g) and SIN-1 (1, 10 and 100 mu g) induced dose-dependent increases in these levels although SNP was much more potent than SIN-1. These results suggest a role of NO in the control of prolactin secretion.