Rats with implanted highly differentiated Dunning R3327PAP prostatic tumors were castrated, and at 3, 7, and 14 days thereafter, the effects on tumor volume, epithelial cell numbers, and sizes were quantified using morphometrical methods. The castration response on these parameters was also examined in the normal prostate of the same rats. Castration resulted in a rapid decrease in organ volume, epithelial cell number, and size in the normal prostate, and morphological signs of epithelial cell death (apoptosis) were observed. Tumor growth and mitotic index were reduced, but there were no signs of increased apoptosis, and cell numbers remained fairly constant in the Dunning tumors during the study period. It is concluded that the castration-induced inhibition of tumor growth is caused by factors other than tumor cell death.