The relative efficacy of hormonal, chemical, or combined therapy was investigated in a prostate cancer animal model. 96 Copenhagen-Fischer hybrid rats were implanted with equal-sized fragments of the Dunning H strain transplantable prostatic adenocarcinoma. These animals were randomly assigned to 1 of 6 treatment groups: (1) control, (2) testosterone, (3) testosterone followed by cyclophosphamide, (4) cyclophosphamide, (5) orchiectomy or (6) orchiectomy followed by cyclophosphamide. Animals in the control group had the shortest survival, fastest tumor growth rate, and largest tumor size. Testosterone, when compared to the control group, did not accelerate tumor growth. Cyclophosphamide alone, orchiectomy alone, cyclophosphamide plus orchiectomy, and testosterone plus cyclophosphamide were each equally effective in decreasing tumor growth rate. Tumor size in the testosterone plus cyclophosphamide-treated animals was equivalent to that in orchiectomized animals. The survival of all groups treated with cyclophosphamide was reduced because of toxicity. Testosterone administered prior to cyclophosphamide eliminated and shortened survival from cyclophosphamide. The best overall results were obtained by orchiectomy alone, orchiectomy plus cyclophosphamide, and testosterone plus cyclophosphamide. Although chemotherapy alone was effective in shrinking tumor size, it was highly toxic.