1. A total of nineteen ram and three goat testes have been perfused in isolation at 34-35 degrees C for 3(1/2)-9 hr with heparinized blood and an added 5-HT antagonist (bromolysergic acid diethylamide) and their function compared with that of the normal ram testes in vivo.2. The metabolism of the perfused testes and the testes in vivo was similar but blood flow through the perfused testes was two to three times normal.3. Vasoconstriction was produced by adrenaline and noradrenaline (10-20 mug I.A.) and by electrical stimulation of nerves in the spermatic cord or of the lumbar sympathetic chain; these responses were abolished or reduced by phenoxybenzamine.4. Flow of fluid from the rete testis continued only if ischaemia was reduced to a minimum and glucose concentration in the blood perfusing the testis was kept above about 25 mg/100 ml.; the fluid secreted during perfusion was of normal composition.5. The perfused testis showed no evidence of autoregulation and the flow of fluid was not affected by changes in perfusion pressure.6. When the temperature of three testes was raised to 40 degrees C for 2 hr, metabolism increased but blood flow was unaltered; the flow of fluid and the concentration of spermatozoa decreased during heating.7. The testes perfused at normal scrotal temperatures (34-35 degrees C) were histologically normal but some abnormalities were observed in the heated testes.