Although increasing data indicate a role for vitamin D in prostate cancer, little is known about the role of this hormone in the noncancerous prostate. We examined the effect of 1,25-dihydroxyvitamin D3 (1,25 D) on the growth of noncancerous rat prostates in vivo. Rats were castrated and treated with vehicle (controls), 1,25 D, testosterone, or a combination of both hormones for 2 weeks. Histological examination of the harvested prostates revealed that 1,25 D had a selective regressive effect on epithelial cells in treated rats compared to untreated castrated rats and to normal uncastrated rats. However, 1,25 D stimulated stromal growth in the prostate. The mean prostatic weight of the vitamin D-treated rats was twice that of the untreated rats (0.13 +/- SEM 0.005 g versus 0.06 +/- SEM 0.006 g). The histological differences were less marked in the testosterone-supplemented animals. A greater degree of cellular differentiation was observed in the rats treated with testosterone and vitamin D compared to rats that received testosterone supplementation alone. Studies of the nuclear matrix composition revealed differences between the testosterone-supplemented and the testosterone and 1,25 D-treated rat prostates. We conclude that in the absence of testosterone, 1,25 D may exert a growth-promoting effect on the prostatic stroma in vivo. In concert with testosterone, it may play an important role in the growth and differentiation of the normal rat prostate.