Explants of canine prostate were cultured in a defined medium for periods of up to 5 days with and without added steroids. Testosterone and 5α-dihydrotestosterone failed to maintain their histological integrity and induced a greatly increased formation of stromal elements. Epitestosterone and 5α-dihydroepitestosterone were partially successful in maintaining epithelial height although secretory activity was not preserved. The only steroid that sustained epithelial height and secretory activity whilst keeping stromal growth at a minimum was 5α-androstane-3α, 17α-diol. The three other epimeric androstanediols were ineffective.