The testosterone level in plasma is measured in 10 males by radioimmunoassay (RIA) at the following times: 07.00 h, 08.00 h, 09.00 h, 10.00 h, 13.00 h, 16.00 h, 19.00 h, 23.00 h and 03.00 h. A circadian rhythm can be noted. Peak levels are reached between 07.00 h and 10.00 h. At 19.00 h testosterone values reach their minimum and rise at night. In the morning between 07.00 h and 10.00 h testosterone levels seem to have a plateau. Between the morning and the evening the values show great differences.