We performed a histological quantitative analysis of collagen and smooth muscle in seminal vesicle (SV) stroma after Estradiol-17 beta (E2) administration to the immature castrated rat. On day 14 after E2 administration, collagen and smooth muscle had increased approximately five and fifteen times, respectively, over the baseline of day 0. On day 28 (14 days after the cessation of E2 administration), smooth muscle decreased to the same level as that on day 7, but collagen did not decrease during the 14 days after the cessation of E2 administration. As the proliferation of stroma, the infiltration of eosinophils was seen in the perigrandular stroma that consisted of rich collagen. The number of infiltrated eosinophils increased exponentially in the period in which collagen proliferated. But in the peripheral stroma which consists of rich smooth muscle, no eosinophil infiltration was seen. These results suggest that eosinophil infiltration into SV is related to E2 administration and to stromal proliferation, especially to collagen proliferation in this condition.