In this study, hypo- and hyper-reproductive conditions, as measured by concentrations of plasma testosterone in male Japanese quail held on long days LD 16:8, were experimentally simulated with injections of 5-hydroxytryptophan (5-HTP) and L-dihydroxyphenylalanine, (L-DOPA) with 8 h and 12 h phase angle differences between them in intact and melatonin-treated birds. The effects of these treatments were assessed on the characteristics of the circadian rhythm in the hypothalamic concentration of serotonin (5-HT), dopamine (DA), and plasma levels of thyroxine (T(4)), triiodothyronine (T(3))(,) and testosterone (T). These rhythms were also studied in sham-operated (SO), pinealectomized (Px), vehicle- (Veh), and melatonin (Mel)-treated birds. On the basis of the circadian mesors of the testosterone rhythms, three distinct categories could be identified: category A (i.e., normal breeding concentrations of plasma testosterone), which includes control, sham-operated, and vehicle-treated groups; category A(+) (i.e., concentrations of plasma testosterone higher than that found in normal breeding quail), which includes 12 h, 12 h+vehicle-treated, and Px quails; and category A(-) (concentrations of plasma testosterone lower than that found in normal breeding quail), which includes 8 h, melatonin-, and 12 h+melatonin-treated groups. It is evident that in normal and hypergonadal conditions (i.e., birds belonging to categories A and A(+)) the circadian rhythm in hypothalamic serotonin maintained a positive phase angle of about 16 h. In contrast, birds of category A(-) (i.e., in a hypogonadal condition) exhibited a negative phase angle of about 2 h. The present results clearly suggest that the internal phase relationship between the circadian rhythms in hypothalamic serotonin and dopamine might play a crucial role in strategizing and conferring a particular reproductive status to the birds. The role of circadian mechanisms involving circulating thyroid hormones in conferring reproductive status is completely ruled out, as no definite internal phase angle between these two hormonal rhythms was witnessed vis-a-vis different treatment groups. The testosterone peaks always occurred at the same time irrespective of breeding status of the bird, but with significant variation in its amplitude (high in hypergonadal and low in hypogonadal condition). It is suggested that administration of 5-HTP and L-DOPA at specific time interval and variation in pineal functions that modulate reproductive responses also alter the circadian pattern (acrophase and amplitude) of hypothalamic serotonin and dopamine, maintaining a specific phase relation between these cycles and breeding status. These findings strengthen our previous reports that a specific circadian phase relation of serotonergic and dopaminergic oscillations regulates reproduction. The present study strongly supports interdependence and specific relation of the two systems (gonadal activity and circadian pattern/phase relation of neural oscillation) in both natural and experimentally simulated conditions.