The notion that circulating leptin may provide a somatic signal for timing the onset of puberty was examined in the male rhesus monkey. Circulating leptin levels were determined at weekly or biweekly intervals by RIA in intact (N = 6) and prepubertally castrated monkeys (N = 5) from approximately 18 to 30 months of age. Circulating testosterone (T) and gonadotropin levels were used as indices to identify the onset of the pubertal reaugmentation of pulsatile GnRH release (week 0) in intact and castrated animals, respectively. Time courses of the peripubertal changes in leptin concentrations in individual monkeys were normalized to week 0. In the intact group, mean leptin concentrations at this critical stage of development (week −26 to week +9) were unremarkable, ranging from 1.6 ± 0.3 (SEM) to 2.4 ± 0.6 ng/ml (P > 0.05). In agonadal males, the pubertal onset of GnRH pulse generator activity, as reflected by sustained increments in plasma gonadotropin concentrations, also occurred in the absence of changes in circulating leptin levels (P > 0.05). These findings indicate that the timing of the onset of puberty in male monkeys is not triggered by rising circulating leptin concentrations.