Gonadal steroids alter the apparent molecular size of intrapituitary Follicle-Stimulating Hormone (FSH) in rats and monkeys as well as increase the percentage of acidic FSH isohormones in sheep. Hence, we hypothesized that the molecular size of ovine (o) FSH would be increased by gonadal steroids. Extracts of pituitaries from rams and wethers, as well as, from wethers which had been implanted with dihydrotestosterone (DHT), 17 beta-estradiol (E2) or both steroids (n = 4-6 per treatment group) were subjected to analytical gel permeation chromatography using Sephadex g-100 Superfine. FSH concentrations in chromatographic fractions were determined by radioimmunoassays. Although FSH in pituitaries of non-implanted wethers eluted slightly earlier (i.e. larger) than FSH in pituitaries from E2-implanted wethers as evaluated by distribution coefficients (Kds) during chromatography (P < 0.05), gonadal steroids did not consistently increase Kds but tended to decrease them. When Kds were extrapolated to apparent molecular weights using a series of standard proteins (bovine serum albumin (bSA), ovalbumin (OA), carbonic anhydrase (CA) and cytochrome c (CC)) that were included in each chromatogram, the differences between treatment groups were not statistically significant (P > 0.05). Thus, in contrast to rats and monkeys, neither castration nor steroid-replacement appears to alter the molecular size of FSH in the sheep pituitary as evaluated by analytical gel permeation chromatography.