ABSTRACT. Pulsatile secretion of LH and FSH was examined in 10 prepubertal (aged 4.5-12.9 y) and seven early pubertal (aged 12.8-14.5 y) boys with ultrasensitive (0.019 and 0.014 IU/L time-resolved immunofluorometric assays. Plasma L H and FSH levels were measured every 15 or 20 min for 6 h during the day and night. The lowest mean LH level in a prepubertal boy was 0.02 IU/L and in eight other prepubertal boys mean LH levels were less than 0.4 IU/L. In early pubertal boys the mean LH levels ranged from 0.3 to 6.5 IU/L. The difference in mean FSH level between prepubertal (0.61 IU/L) and early pubertal boys (1.85 IU/L) was smaller than the difference in LH level. All boys had significant LH and FSH pulses. The LH interpulse interval was 135 f 86 min (mean f SD) and 76 f 65 min for the prepubertal and pubertal boys, respectively (p < 0.01). For FSH, the respective values were 150 f 122 and 221 f 157 min ( p = NS). The mean L H pulse amplitudes were 11-fold greater in the early pubertal boys than in the prepubertal boys, whereas the mean FSH pulse amplitudes were similar between the two groups. The present method shows that the mean L H levels in prepubertal boys are much lower, and the increase during puberty larger, than previously reported. The increase is apparently due to increased pulse frequency and amplitude. The increase in mean FSH level is smaller and evidently not caused by an increase in pulse frequency or pulse amplitude. (Pediatr Res 27:215-219,1990) Abbreviations