After a single PMS (50IU) injection to 25-day-old rats, FSH receptor content of the ovarian tissue increased progressively for4days, then showed a tendency to decrease, while LH receptor content remained unchanged for 3 days, then gradually increased. From these facts, we established a radioreceptor assay system, employing 3, 000rpm precipitates of homogenates of the ovaries obtained3days after PMS injection as the receptor preparation. The dissociation constant of the binding of the receptor preparation to rat FSH was 7.2×10-11M. The standard curve was obtained with 0.125-16ng/tube of NIAMDD rat FSH1-3. Purified preparations, NIAMDD rat LH I-4 and NIAMDD rat TSH I-4influenced the binding only at high concentrations possibly owing to FSH contamination. When the anterior pituitary homogenates obtained from rats in the various physiological states were assayed by this system, the intra-assay coefficient of variation and inter-assay coefficient of variation were 7.5 % and13.7%, respectively, and the assay values were well correlated with those obtained by radioimmunoassay (r=0.988, the slope of the regression line=1.14).