Plasma concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), 17 alpha-hydroxyprogesterone (17 alpha-OHP), 17 beta-oestradiol (Oe2) and progesterone (P) were measured in 352 healthy girls aged 6.0 to 15.9 years, as a function of age (CA), weight, height and calculated lean body mass (LBM) and total body fat (TBF). The earliest hormonal changes were a fall in mean plasma FSH concentrations together with a small but significant rise in plasma Oe2 well in advance of any sign of pubertal development. The next changes were a progressive rise in plasma FSH and 17 alpha-OHP concentrations without further increments in plasma Oe2; these changes corresponded to a mean body weight of 29.8 kg, a mean height of 132.0 cm (initiation of the adolescent growth spurt), a mean LBM of 24.7 kg and a mean TBF of 5.1 kg. The last events were a progressive rise in plasma LH and Oe2 and less marked in P, which occurred in association with a mean body weight of 40.0 kg, a mean height of 142.0 cm (time of peak velocity of weight and height gain), a mean LBM of 31.8 kg and a mean TBF of 9.1 kg. Significant quadratic equations were disclosed between plasma FSH and LH versus CA, weight, height and LBM, and a significant linear correlation was observed between each gonadotrophin and TBF. These results show an association, not necessarily causal, between a 'critical level' of body composition and hormonal changes at the start of the adolescent growth spurt, as well as with late hormonal events at the time of peak velocity in weight and height gain. On the other hand, LBM rather than TBF seems more closely associated with the initiation and progression of puberty.