The blood flowing through the internal spermatic artery of the ram has been shown to cool by 5.2[degree]C (range 4.5[degree] to 5.8[degree]C) between the aorta and the dorsal pole of the testis when testicular temperatures are 33.3[degree] to 34.7[degree]C. Most of the cooling occurs in the coiled portion of the artery in the spermatic cord where the venous blood returning from the testis through the pampini-form plexus warms by a similar amount. It is concluded that the vascular arrangement in the spermatic cord of the ram is a remarkably efficient heat-exchange system. The temperature deep in the testis remained close to that of the inside of the scrotal skin throughout controlled variations in the range 28[degree] to 40[degree]C. Changes in the temperature of the scrotum are quickly transferred to the blood in the superficial veins of the testis which then, by heat exchange in the spermatic cord, alters the temperature of the testicular arterial inflow. It is concluded that the vascular mechanism is not by itself regulatory but it rapidly transfers any benefit of scrotal thermoregulation to all parts of the testis.