The effects of continuous i.v. infusion of porcine follicular fluid (PFF), presumed to contain inhibin activity, on plasma gonadotropin levels were investigated in adult male and female Sprague-Dawley rats. One group each of male and female rats was castrated just prior to the start of treatment of 10-day duration (ACUTE). Another ACUTE group of females was treated for 30 days. A final group of female rats was ovariectomized 3 months before the start of treatment of 10-day duration (CHRONIC). When administered at a dosage of 1.0 mg/100 g body weight/24 h, charcoal-extracted PFF suppressed plasma follicle-stimulating hormone (FSH) concentrations to hypophysectomy levels within 48 h and maintained these levels for 10 days in all groups treated. In the ACUTE group of female rats treated for 30 days, FSH suppression was maintained during infusion up to 20 days. This inhibition became attenuated after 25 days of treatment and completely disappeared by 30 days. There was no discernible effect of PFF on luteinizing hormone (LH) concentrations at any time. Upon cessation of PFF treatment at 10 days in both ACUTE and CHRONIC ovariectomized rats, FSH levels rebounded to normal ovariectomy control concentrations within 2-4 days. Porcine serum (PS) was similarly extracted and administered as a control substance. It exerted no detectable effect on either FSH or LH concentrations in ACUTE or CHRONIC castrated rats. These results demonstrate that PFF alone is capable of maximally suppressing FSH secretion when administered by continuous infusion. This finding is especially significant considering that such a pronounced effect was obtained in the total absence of gonadal steroids.