Circulating GH levels measured by RIA in developing female rats were elevated between day 5 and 12. Titers declined by day 15, reaching a nadir at day 20 and by day 25 they began to increase again. During the pubertal period (days 32-38), plasma GH was elevated in the anestrous phase, reaching peak values in the afternoon of the early proestrous phase. Levels were also elevated the day of the first preovulatory discharge of gonadotropins, showing peak values in the afternoon of that day. Thereafter, plasma GH declined, reaching a nadir in the morning of diestrus 1 (day 2 after vaginal opening) and remained slightly more elevated during the afternoon of that day and throughout diestrus 2. Plasma GH levels at this time were similar to those of adult diestrus females. Pituitary GH concentration was low between days 5 and 15 and increased markedly thereafter, reaching adult concentrations by day 25. Body weight increased linearly between day 5 and 25. At this time, the slope of the curve of body weight gain increased significantly. Although the majority of animals revealed open vaginae only after they had reached a body weight of 100 g, a critical weight for vaginal opening could not be established. The pronounced changes in circulating GH levels which preceded by several days the surge in gonadotropin release that normally accompanies the onset of puberty in the rat suggest that GH may play a physiological role in the control of the pubertal process in this species.