To clarify the effect of androgen on the microcirculation in seminal vesicles of adult Wistar rats, we investigated the organ blood flow and morphological features in the capillaries after castration and subsequent testosterone supplementation. Testosterone (T) was subcutaneously injected every 12 hours after castration and its doses were set to 10(-2), 10(-1), 10(0) and 10(1) mg/kg-body weight (T10(-2), T10(-1), T10(0) and T10(1) groups, respectively). Organ blood flow was measured using laser Doppler flowmetry, and the subepithelial capillaries were analyzed using transmission electron microscopy. The capillaries were morphologically classified into 3 types; oval opened (type 1), intermediate (type 2) and collapsed (type 3), and their luminal areas were measured using a computed image analyzer. The organ blood flow was significantly reduced from 36.3+/-5.1 to 21.9+/-2.7 ml x min(-1)/100 g tissue, and the luminal area of the capillaries was significantly reduced from 9.02+/-1.28 to 4.85+/-0.82 microm2 with the shift of the type 1 and type 2 to type 3 after castration. The reduction of the luminal area and the blood flow reduction, and shift of the capillary type were significantly protected by gradated testosterone supplementation. These results indicate that the maintenance of the blood flow and morphological profiles in capillaries depend on androgen-supplementation levels in seminal vesicles.