The efficiency of countercurrent transfer of heat and 133Xenon (133Xn) in spermatic cords of the bull was measured under in-vitro conditions. Plasma was pumped through the artery and veins via two separate systems under controlled temperature conditions. Cold plasma or 133Xn-saline was introduced into the afferent veins and the transfer of heat or gas from the venous to the arterial side was measured. The mean efficiency of heat transfer was found to be 91% in five experiments. In four experiments a mean of 12% of the radioactive gas introduced into the venous system was recovered from the arterial side. It is concluded that the countercurrent transfer system in the spermatic cord is very effective with respect to the transfer of heat and probably moderately effective in the transfer of inert gases. The transfer is probably due to diffusion as eventual transfer through arterio-venous shunts was minimal.