D1-dopamine receptor stimulation induces c-fos gene expression in the fetal suprachiasmatic nucleus (SCN), but not in the adult rat SCN. Light exposure at night induces c-fos gene expression in the adult SCN. The present experiments were designed to define the developmental transition from dopaminergic to photic regulation of c-fos gene expression in the SCN. Treatment with the D1-dopamine receptor agonist SKF 38393 (10 mg/kg) increased SCN c-fos gene expression during both day and night on postnatal day (PD) 0, 1, and at night on PD 2, but the c-fos response disappeared by PD 4. Photic induction of c-fos gene expression was apparent during both day and night at each age examined, from PD 0 through PD 6. The magnitude and distribution of c-fos expression following light during the daytime was distinguishable from the response to light at night beginning on PD 2, indicating that the circadian clock regulates (gates) the c-fos response by PD 2. Orbital enucleation prevents the induction of c-fos by light at night on PD 2, indicating retinal mediation. Developmental loss of the c-fos response to SKF is not precipitated by the arrival of the retinohypothalamic tract; animals enucleated on PD 0 were insensitive to SKF on PD 6, as were visually intact controls. The results demonstrate that both dopaminergic and photic inputs can regulate SCN c-fos gene expression early in the neonatal period, and that the developmental loss of sensitivity to SKF is not due to the arrival of photic input to the SCN. The developmental transition from dopaminergic to photic regulation of c-fos gene expression roughly parallels the developmental transition from maternal to photic entrainment of the developing biological clock.