The somatic manifestations of puberty in a cohort of 51 boys were correlated with changes in the concentrations of plasma testosterone, LH, and FSH. The stage of pubertal development was graded (P1–P5), testicular size measured, and skeletal maturation determined. The mean concentration of plasma testosterone increased from 19 ng/100 ml ± 4.4 (sem) in P1, to 71 ng/100 ml ± 19 (sem) in P2, to 248 ng/100 ml ± 46 (sem) in P3, and to 482 ng/100 ml rfc 27 (sem) in P4–5. The mean concentration of plasma FSH (LER 869–2) rose from 0.8 ng/ml ± 0.05 (sem) in P1, to 0.96 ng/ml ± 0.15 (sem) in P2, to 1.7 ng/ml ± 0.3 (sem) in P3, to 2.5 ng/ml ± 0. 5 (sem) in P4–5. The mean concentration of plasma LH (LER 960) rose from 1.1 ng/ml ± 0.06 (sem) in PI, to 1.5 ng/ml ± 0.15 (sem) in P2, to 1.6 ng/ml ± 0.07 (sem) in P3, to 1.7 ng/ml ± 0.11 (sem) in P4–5. A considerable variation in plasma hormone values was noted in P2 and P3. The testicular volume index increased from 1.7 cm2 ± 0.1 (sem) in Pi, to 4.3 cm2 ± 0.6 (sem) in P2,...