Ablation of the suprachiasmatic nucleus (SCN) at 2 days of age, prior to the formation of the retinohypothalamic projection, produces a permanent state of constant vaginal estrus in the postpubertal female rat. Although such lesions do not alter the onset of puberty in sighted rats, they do compensate for the delay in vaginal opening induced by neonatal binding. The ovaries of sighted and blinded SCN-lesion rats are small and polyfollicular and the pituitaries of blinded SCN-lesion rats are abnormally large. Sampling of plasma in the morning and afternoon for up to 12 consecutive days in sighted SCN-lesion rats reveals continuously low luteinizing hormone levels. This constellation of endocrine alterations does not correlate with damage to any structures outside the SCN. Since the organization of the rodent estrous cycle is circadian, these results emphasize further the importance of the SCN in circadian rhythm generation. The necessity of an intact SCN for the development of normal, cyclic reproductive function implies that sparing or recovery of function does not occur.