In order to determine whether there are variations in pituitary FSH [follicle-stimulating hormone] content that can be correlated with sexual maturation, female Sprague-Dawley rats were sacrificed at 10, 15, 20, 25, 30, 35 and 40 days of age and the FSH content of their pituitaries measured by the Steelman-Pohley assay. The values for total FSH content per pituitary in terms of standard NIH-FSH-S-3 for each of the respective ages were 43.4, 61.7, 163.8, 124.0, 86.0, 30.1 and 23.0 [mu]g. The FSH concentrations per mg of wet tissues were 31.0, 34.3, 77.1, 45.6, 22.3, 7.0 and 4.0 [mu]g, respectively. The vaginas were open in 27% the 35 day-old rats and in 87% of the 40-day-old rats. In a group of 38-day-old rats, the mean pituitary FSH content was 54.3 [mu]g in those with closed vaginas and 51.3 [mu]g in those with open vaginas. Thus, FSH content of the pituitary rises to a peak prepuberally at the age of 20 days and declines thereafter without any significant acute change at the time of vaginal opening.