Abnormal angiogenesis underlies many pathological conditions and is critical for the growth and maintenance of various types of tumors, including hormone-dependent cancers. Since estrogens are potent carcinogens in humans and rodents, and are involved in regulating angiogenesis, this study was designed to examine the effect of estrogen on the behavior of an established bovine capillary endothelial cell line, a simple and physiologically relevant model of the capillary wall. The results demonstrate that 17beta-estradiol (E2), at different conditions, exerts both stimulatory and inhibitory effects on endothelial cell adhesion, proliferation and tube formation in vitro. Utilizing a cellular attachment assay, chronic exposure to nanomolar concentrations of E2 (i.e. 1 and 10 nM) increased endothelial cell adhesion significantly compared to vehicle treated controls. Cellular adhesion was inhibited by micromolar concentrations of E2. Cell count, PCNA immunohistochemistry and Western blot analysis demonstrated enhanced cell proliferation at low E2 concentration in estrogen-deplete medium. Inhibition of cellular proliferation was observed in both estrogen-replete and deplete medium at higher E2 concentrations (i.e. 1 and 10 microM). Furthermore, in vitro tube formation increased up to 3.0 fold in the presence of 10 nM and higher E2 concentrations. The present observations indicate that in vitro regulation of capillary endothelial cell adhesion, proliferation and capillary tube formation by estrogen, are dose dependent.