The concentrations of LH and FSH were determined in serum samples obtained at 15 min intervals during a 4 h period from seven normal girls, 9.5-16.5 years of age, and eight normal boys, 9.0-16.8 years of age. An episodic pattern of LH secretion was found in all subjects. There was no significant (P greater than 0.1) difference between girls and boys as to number of episodes (3.0 +/- 0.6 vs. 3.0 +/- 0.0 per 4 h), absolute LH increase (2.6 +/- 1.1 vs. 2.4 +/- 1.0 mIU per ml) or "apparent LH half-life" (64.3 +/- 20.8 vs. 76.1 +/- 23.4 min). Mean absolute increase during secretory episodes correlated (R = 0.89, P less than 0.005) with the mean LH concentration per 4 h, and a significant (P less than 0.025) inverse correlation (R = -0.52) was found between the mean per cent increment of the LH secretory episode and the mean "apparent LH half-life." Minor fluctuations of FSH were also observed. In girls and boys, the mean concentration of both gonadotropins increased with advancing puberty. The increase in LH concentration was due to an increase in the amplitude of secretory episodes rather than to an increase in the number of episodes. The increase in FSH concentration reflected a consistent elevation throughout the period of sampling.