Administration of 100 μg of testosterone (T) daily for 14 and 28 days to 7-day castrate rats restored the weight of the ventral prostate to a level which slightly exceeded that of the controls. Ventral prostate weight in groups receiving estradiol-17β (E2) doses of 10, 50, 100, 200, or 500 μg administered simultaneously with 100 μg of T did not differ significantly from intact controls, although the weights were lower at E2 levels greater than 100 μg. Body weights of the castrated rats receiving 100 μg of T did not differ from those of sham castrated controls. However, mean body weights of all groups which received E2 (10 to 500 μg) simultaneously with 100 μg of T were significantly less than (p