Testicular blood flow and testosterone concentrations in the spermatic venous plasma were measured on unilaterally cryptorchid rats. Blood flow to the cryptorchid testis was 31.4 +/- 11.7 (SD) ml/100 g X min which was significantly higher thant that of the scrotal testis (17.7 +/- 4.4 ml/100 g X min). Stereological analysis showed a relative increase of blood vessel containing interstitial tissue in the cryptorchid testis, which was probably the main factor responsible for the relative increase of blood flow to the cryptorchid testis. The increase of interstitial tissue was greater than the increase of blood vessels and thus, the interstitium in the cryptorchid testis contained a number of vessels which was smaller than that of the interstitium in the scrotally located testis. The concentration of testosterone in the spermatic vein of the abdominal testis was 18.0 +/- 5.5 (SD) ng/ml and the corresponding value for the scrotal testis was 41.2 +/- 7.0 ng/ml. Calculations based on functional and morphological data indicate that the function of the Leydig cells in the abdominal testis was impaired. It was concluded that the outflow of testosterone from the cryptorchid testis was highly reduced.