• The administration of a standard course of testosterone in a series of subfertile male patients resulted, as a rule, in the disappearance of spermatozoa from the semen after 12 weeks. After termination of this treatment, spermatozoa reappeared in two or three months. While in some cases the sperm counts returned to the pretratment level, in others they failed to reach it during the period of observation and in some they exceeded it (rebound phenomenon). The selection of patients must be governed by determinations of urinary gonadotropin excretion and by testicular biopsy, especially the latter. In one initially azoospermic patient the rebound phenomenon was observed twice and a peak count of 30 million spermatozoa per cubic centimeter was achieved. In this instance a pregnancy followed, with subsequent normal childbirth.