Nitric oxide (NO), a highly reactive and short-lived radical, is considered to be an important trigger molecule for several physiological mechanisms including gonadotrophin releasing hormone (GnRH) secretion in mammals, although there is no such information in avian literature. On the other hand, specific temporal phase relation of circadian neural (serotonergic and dopaminergic) oscillations is reported to modulate reproductive activity in many avian species including Japanese quail. The present study was undertaken to investigate the correlation of NO activity and gonadal function of Japanese quail. In experiment I, the effect of serotonin and dopamine precursors, (5-hydroxytryptophan (5-HTP) and L-dihydroxyphenyalanine (L-DOPA) respectively; 5 mg per 100g body weight) administered at intervals of 8 or 12h over a period of 13 days, was studied on reproductive responses and NO activity. Measurements of body weight, cloacal gland size, testosterone concentration, spermatogenesis, nitrite-nitrate concentration in plasma, hypothalamus and testes, and NADPH-diaphorase (NADPH-d) activity in testes were made on the 2nd, 3rd, 6th and 11th days of treatment and 2nd and 30th day post-treatment. In experiment II, quail were divided into five groups including the control. One experimental group received 13 daily injections of 5-HTP and L-DOPA at intervals of 8h along with 0.1 ml of normal saline administered orally (8-hr+Veh), while another group of 8-hr quail received NO donor (sodium nitroprusside (SNP), 5 mg per 100 g body weight) orally (8-hr+SNP). The third experimental group received 5-HTP and L-DOPA at intervals of 12h along with normal saline (12-hr+Veh), while the fourth group of quail along with 5-HTP and L-DOPA at intervals of 12h also received the NOS inhibitor (N-nitro-L-arginine methyl ester, L-NAME, 25 microg per 100 g body weight) intraperitoneally (12-hr+L-NAME) for 13 days. This experiment was terminated after 21 days of the treatment. Results indicate that 5-HTP and L-DOPA administered 8h apart (8-hr) suppressed but if given 12h apart (12-hr) stimulated the reproductive system and NO activity compared to the control. These effects were apparent on the 6th day of injections and were maintained 30 days following the termination of the treatment. A significant decrease in nitrite and nitrate concentration and NADPH-d activity in reproductively inhibited 8-hr group and an increase in reproductively stimulated 12-hr quail was also evident. In contrast, these activities were stimulated in 8-hr+SNP quail and were suppressed in 12-hr+L-NAME group quail. It is concluded that activity of the reproductive system and NO activity waxes and wanes simultaneously in Japanese quail. Moreover, experimental modulation of gonadal activity (following changes in the phase relation of serotonergic and dopaminergic activity) or NO activity (following the administration of NO modulator or inhibitor) affects each other maintaining a parallel relation between the two systems. Further, it is interesting to note that the gonado-stimulatory effect of SNP overpowers the gonado-inhibitory effects of the 8-hr time interval and inhibitory effects of L-NAME mask the stimulatory effects of 12-hr temporal relation of 5-HTP and L-DOPA administration. These findings strongly suggest that reproductive effects may be induced via changes in NO activity, however the exact mechanism by which NO drives gonadal axis needs to be ascertained.