In a morphometric study on the ventral prostate and seminal vesicles in the rat, we investigated the changes in fibromuscular stroma, glandular epithelium, and glandular lumen. Animals were studied at 15, 30 and 45 days of age. The rapid prepubertal growth started earlier in the ventral prostate than in seminal vesicles. In addition, the effects of neonatal administration of estrogens on the different tissue compartments were studied, comparing rats that had been castrated and/or treated with estrogen at birth to intact animals at 15 days of age. Estrogens caused a decrease in the volume of the glandular epithelium and increased the volume of the fibromuscular stroma in both ventral prostate and seminal vesicles. Castration partially abolished the estrogen-induced growth of the stroma, which suggests that the growth is dependent on testicular factors. The difference in proportion of the fibromuscular stroma between the two glands is evidence that the size of the whole seminal vesicles has increased whereas the size of the ventral prostate has decreased.