Sequential blood samples were obtained from undisturbed, unrestrained, male and female rats of different ages from intrajugular or intraaortic cannulae implanted 4 days before experimentation. Blood (70–100 μl) was gently withdrawn at 30-min intervals for periods of 6 h. Plasma GH was determined by RIA.

In 22-day-old rats, episodic secretion of GH was evident, but plasma GH levels did not exceed 70 ng/ml. In older rats, peak levels of GH were higher and there was a significant difference in the secretory pattern of GH between male and female rats. In male rats, 30, 45, and 90 days old, GH surges in individual animals occurred at regular 3- to 4-h intervals. Peak levels were 200-300 ng/ml and levels between peaks, were mostly <5 ng/ml. The timing of the peaks with respect to clock time was similar in most animals. In 30-day-old rats, peak levels were lower in females than in males. In 45-day-old female rats, GH was secreted episodically, and peak levels were 200-300 ng/ml. However, the interval between peaks was shorter than that in males, and there was no apparent timing of the peaks among these animals. In 90-day-old female rats, peaks occurred at irregular intervals with no apparent timing among the animals. Between peaks, GH levels were higher than in male and younger female rats.

The development of sex differences in the GH secretory pattern occurs at about the time of changes in growth rate between males and females (25–30 days), although there was no apparent correlation between GH levels and rate of body weight gain after the onset of puberty.