Forty-six boys were followed at 6-month intervals up to 48 months during the years of puberty with examinations to evaluate the extent of sexual development and with serum samplings to determine LH, FSH, testosterone and prolactin concentrations. LH levels rose throughout puberty, the initial rise of LH occurring before testosterone concentrations were significantly increased. Testosterone levels rose progressively from before the appearance of sexual hair until an adult distribution was achieved. Testosterone concentrations did not increase until after growth of the genitalia was appreciated while genital growth continued after elevations of testosterone were no longer detectable. Levels of FSH rose before sexual hair appeared but after the initial enlargement of the testes occurred. No further elevation of FSH occurred after mid-puberty. Prolactin concentrations did not change throughout puberty.