The LH surge was induced in ovariectomized rats by sequential treatment with oestradiol benzoate and progesterone. Vasoactive intestinal peptide (VIP) or saline was infused into the third cerebral ventricle from 13.30 to 16.30 h on the afternoon of the anticipated LH surge. Two blood samples were taken by jugular puncture from each animal, one at 12.00 h as a control sample and the other at 16.00, 18.00, 20.00 or 22.00 h. Saline-infused animals showed a normal LH surge, with mean plasma LH concentrations reaching a peak at 18.00 h, declining by 20.00 h and reaching control (12.00 h) levels by 22.00 h. Plasma LH in animals infused with VIP was not significantly higher than control levels at 16.00 or 18.00 h. By 20.00 h, mean LH levels in VIP-infused rats had risen to the levels seen at that time in saline-infused rats, and by 22.00 h LH had returned to control levels in VIP-infused animals. We interpret these findings to mean that VIP inhibits LH secretion during the LH surge. It does not block the surge completely, as pentobarbital during the critical period would have done; nor does VIP appear to affect the timing of the LH surge. Rather, VIP inhibits the increased LH secretion rates of the LH surge only during the period of VIP treatment and for a short time afterward.