1 microgram/100 g body weight of synthetic luteinizing hormone-releasing hormone (LHRH) was administered to immature (25, 35 and 55 days of age) and adult (180 days) male rabbits. Testosterone was measured by radioimmunoassay in plasma samples taken just before and 30 min after LHRH injection. At all stages studied, LHRH significantly increased plasma testosterone. Expressed as a percent of over basal levels, the plasma testosterone was increased by 2,695% at 25 days, 3,955% at 35 days, 6,032% at 55 days and 3,766% at 180 days. Expressed in absolute amounts, the plasma testosterone levels showed an increase of 520 pg/ml at 25 days, 604 pg/ml at 35 days, 1,189 pg/ml at 55 days and 7,596 pg/ml at 180 days. The exact interpretation of the data was difficult since it is dependent on the mode of expression used. Nevertheless, the results support the observation that the hypophyso-testicular axis is functional in immature rabbits.