Changes of systemic and regional hemodynamic parameters of the rat during heating of the lower part of the body in 41 degrees C water and after removing of the testis from abdominal cavity, were measured with labeled microsphere technique. Experimental cryptorchism reduced the blood pressure due to a decrease in total peripheral resistance. Hyperthermia of the testis increased the heart rate. Heating of the scrotal region led to an increase in the blood flow that did not, however, reach the value of the blood flow in testis of the control unrestrained rats, thus suggesting an inadequate blood supply of the testis during heating. The inadequate blood supply of the testis under hyperthermia may be one of the factors leading to lesions of the testis.