University of Oxford, Nuffield Institute for Medical Research, and John Radcliffe Hospital, Department of Obstetrics & Gynaecology, Headley Way, Headington, Oxford, 0X3 9DU (Received 9 March 1976) Setchell, Shackleton & Himsworth (1975) recently reported the effects of the sedative Sernylan (phencyclidine hydrochloride) on the concentration of cortisol in the plasma of non-pregnant rhesus monkeys. It was found that after an early decline, plasma cortisol increased gradually during a 4 - 5 h period of prolonged sedation. In our department, some non-pregnant monkeys are routinely bled without anaesthesia. These animals have been trained to jump from their cage into an aluminium box with an adjustable foam-lined lid, which allows them to be restrained gently during sampling from the saphenous vein. The present communication compares the plasma cortisol concentrations in monkeys that had become used to this sampling procedure over several months, with values measured in unanaesthetized animals that had not previously