The effects on reproductive function of restriction to one half Of the normal food intake for 30 or 60 days and subsequent unrestricted feeding were investigated in adult female Sprague-Dawley rats. Restricted feeding resulted in the cessation of oestrous cycles within 9 to 28 days, associated with decreased pituitary, ovarian and uterine weights. Pituitary content and concentration of FSH were increased by restricted feeding and the levels of FSH in 60 day underfed rats were about twice as high as those of the controls at dioestrus. There was little difference in pituitary LH content between the underfed groups and the controls at the end of restricted feeding. Pituitary LH concentration was significantly higher in rats underfed for 60 days than in the control rats. Pituitary prolactin content was one half and one third of that in control rats in rats underfed for 30 and 60 days respectively. Pituitary prolactin concentration was also decreased by restricted feeding. At the end of restricted feeding, no differences in serum LH and prolactin levels were found between the groups, whereas the serum FSH level in 60 day underfed rats was higher than that in the controls at dioestrus. After re-feeding, normal oestrous cycles returned within 3-5 days in almost all rats, regardless of the length of the previous cessation of oestrous cycles. Ptuitary contents of FSH and LH in underfed rats decreased after re-feeding following the return of oestrous cycles. The rate of decrease was much greater in 60 day underfed rats than that in 30 day underfed rats. On the other hand, serum levels of FSH, LH and prolactin in these underfed rats were increased by re-feeding and the levels in the evening of the first pro-oestrus were higher than those in the morning of dioestrus and pro-oestrus. Serum levels of these hormones increased more in 60 day underfed rats than in the other groups at any stage. After re-feeding, pituitary, ovarian and uterine weights increased and the uterine epithelial layer was clearly repaired in both underfed groups, although not always to the control levels by the first oestrus. The end-bud system of the mammary gland which degenerated during restricted feeding was comparable to that of the controls at the first oestrus after re-feeding in 30 day underfed rats, but not in 60 day underfed rats.