The administration of 1200 i.u. pregnant mares' serum gonadotrophin (PMSG) to female goats 2 days before withdrawal of intravaginal progestagen-impregnated sponge led to a biphasic ovulatory response such that up to several ovulation points were present 'prematurely' by the time of sponge withdrawal, followed by a 'superovulatory' wave of ovulations appearing approximately 2 days after sponge withdrawal. This phenomenon could not be prevented by the administration of increased doses of progestagens, via either two fresh intravaginal sponges, or i.m. progesterone in oil (50 mg daily or 50 mg four times per day). Plasma was sampled from ten goats at 4 h intervals following the administration of PMSG. Eight of the goats had ovulations present within 72 h of the administration of PMSG, but luteinizing hormone (LH) concentrations, determined by radioimmunoassay, were less than 1.5 ng ml-1 throughout this period, indicating that the premature ovulations were not preceded by surges in the release of endogenous LH. When the concentrations of LH in the samples from two of these goats were further determined by bioassay, they were found to have increased from less than 1 ng ml-1 at the time of administration of PMSG to greater than 5 ng ml-1 within 12 h, whereafter they remained elevated for a further 72 h. Thus, we concluded that LH bioactivity inherent within PMSG can directly induce ovulation.
