All neural connections entering the hypothalamus from the anterior direction were severed by means of a Halász knife-cut in female rats at 22 days of age. The rats were then examined for vaginal opening and ovulation. At intervals blood samples were collected by cardiac puncture for the measurement of serum corticosterone (B). Unoperated rats showed vaginal opening at 36.5 ± 2.1 days and ovulation within 2 days. A significant difference was found between B at 08.00 h and 18.00 h at the first sample interval chosen (28 days of age) and thereafter. Animals given sham surgery showed vaginal opening late (44.2 ± 3.2 days, p < 0.05) but ovulated within 2 days. These animals also had a significant 08.00–18.00 difference in B from day 28 on. Rats with frontal cuts in front of the suprachiasmatic nucleus (SCN) showed precocious vaginal opening (27.2 ± 2.3 days, p < 0.05) and early ovulation (28.2 ± 0.7 days). A 08.00–18.00 difference was noted at 28 days of age but was lost after ovariectomy at 47 days of age. Implantation of a silastic estradiol benzoate capsule restored the 08.00–18.00 difference (p < 0.01). Animals with cuts behind the optic chiasm (posterior frontal cuts) all showed early vaginal opening; some rats then ovulated (at 42.3 ± 3.8 days) while others did not. No difference could be detected in the position of the cuts although there appeared to be more damage to the region just behind the SCN in the latter group. No 08.00–18.00 difference in B was found in the nonovulatory group and no difference was found in the ovulatory group until 44 days of age. It can be concluded that an isolated hypothalamic island containing the SCN will generate signals sufficient for precocious ovulation although subsequent ovulatory cycles ar