Total plasma androgens (PA) were measured in 9 hypogonadal males aged between 13 and 21 1/2 years after a single oral dose of testosterone undecanoate (TU). With the exception of one patient, all showed a rise in PA with peak values between 7.7 and 38.0 nmol/l at 2 to 7 hours. A further patient aged 15.7 years who was given an 80-mg dose had a peak PA level of 71.2 nmol/l. In all patients PA returned to basal levels at 24 hours. In 4 patients plasma testosterone and dihydrotestosterone were measured, both rose after oral TU, and relatively high plasma dihydrotestosterone values were obtained. While these results indicate that TU is effective in young people, the very high peak androgen levels found in several of them after 40 mg TU suggest that this dose may be excessive in patients in whom growth is not complete.