Anterior pituitary glands were removed from ovariectomized hamsters after specific endocrine treatments. The presence and relative proportions of the multiple species of FSH present within them were assessed by radioimmunoassay after separation by chromatofocusing. This technique is superior to polyacrylamide gel-isoelectric focusing, as it can accommodate a greater sample volume and has increased resolution. Endocrine conditions which decreased hypothalamic LH releasing hormone (LHRH) release or the sensitivity of the pituitary gland to that neurohormone (phenobarbitone treatment or short-term oestradiol exposure) caused an increase in the relative proportions of the more acidic forms (isoelectric points (pI) 5.1-3.8) of pituitary FSH and a concomitant reduction in the more basic (pI values 6.0-5.3) forms of FSH. During times of increased pituitary LHRH exposure (immediately before the oestradiol-induced gonadotrophin surge or after injection of synthetic LHRH) an increase was observed in the relative proportion of the more basic forms of pituitary FSH. Treatment of ovariectomized hamsters with an inhibin-containing preparation reduced serum FSH concentrations as well as the relative proportion of the more basic forms of pituitary FSH. We have previously shown that the more studies suggest that the existing hormonal milieu, in particular LHRH and oestradiol, influences the types of FSH produced and (presumably) secreted. Thus, through hormonal interactions, the pituitary gland regulates not only the absolute amount but also the potency of the FSH signal to the ovaries.