An animal preparation has been developed which permits investigation of steroid secretion by the prostate of the dog. In this preparation the prostate is removed from the dog, placed in a metabolic chamber and infused with the animal's arterial blood via the prostatic arteries at a constant rate. This animal preparation secretes 5α-dihydrotestosterone into the prostatic venous blood. While testosterone is the most probable precursor for secreted 5α-dihydrotestosterone, data from experiments indicate that other steroids may serve as precursors for 5α-dihydrotestosterone in this animal preparation in vivo. The presence of »storage pools« and »secretory pools« for steroids in the infused canine prostate has been discussed.