Previous work in this laboratory indicates that in ovarian tissues correlations can be made between the functional state of the tissues and fluctuations in activity of such enzymes as succinic dehydrogenase (SDH-ase) (Meyer, Soukup, McShan, Biddulph, 1947), malic dehydrogenase and cytochrome oxidage (McShan, Erway, Meyer, 1948), adenosine triphosphatase (Biddulph, Meyer, McShan, 1946), and acid and alkaline phosphatase (Stafford, McShan, Meyer, 1947). The experiments to be reported here were undertaken to determine whether such a relationship exists in the prostate and seminal vesicle with respect to SDH-ase and cytochrome oxidase following.castration and replacement therapy. materials and methods The animals used were Sprague-Dawley male rats ranging from 200 to 300 gm. body weight. Of a total of 55 animals, 7 were set aside as normal controls and did not receive any treatment. One animal was used for each experiment; 5 for SDH-ase controls and 2 for cytochrome oxidase. The remainder were castrated and divided into 3 groups.