Physiological angiogenesis occurs in female reproductive organs, in growing antlers as a result of long-term exposure to cold and possibly hypoxia, and due to increased activity (training) in skeletal and cardiac muscle. The common denominator is increased blood flow, which may result in increased velocity of flow and/or diameters in arterioles and capillaries, increased capillary pressure and increased capillary hematocrit. Increased velocity would lead to increased shear stress, while increased pressure and/or diameters would increase wall tension. Either of these factors may cause a disturbance of the endothelium on the luminal side of vessels. In addition, increased contractile activity during training could cause changes on the abluminal side (for example, modification of the capillary basement membrane or the extracellular matrix induced by stretch/relaxation). In order to elucidate the role of these individual factors in angiogenesis, microcirculation was studied in skeletal muscles which were exposed to: (a) increased activity by chronic electrical stimulations; (b) long-term increase in blood flow by various vasodilators; (c) long-term administration of CoCl2 to increase hematocrit; and (d) long-term stretch, achieved by removal of agonist muscles. Capillary growth, demonstrated as an increased capillary/fiber ratio, as determined by histochemical staining and by electron microscopy, occurred in (a), (b), and (d), but not (c). Capillary proliferation, estimated by labeling index for bromodeoxyuridine of capillary-linked nuclei, occurred in (a), but not in (b). Chronic electrical stimulation resulted in an increase in the diameter of capillaries, a transient widening of arterioles, and no change in venules. Capillary hematocrit and the velocity of red blood cells (Vrbc) were also increased. Calculated shear stress and capillary wall tension were higher in stimulated muscles than in control muscles. Long-term increase in blood flow, induced by administration of the alpha 1-blocker prazosin, caused increased Vrbc with no change in diameters and increased only capillary shear stress. Stretched muscles had decreased blood flow, but longer sarcomeres initially caused concomitant stretch of capillaries. Increased shear stress/wall tension/stretch may initiate angiogenesis by damaging the luminal side of endothelial cells and/or their basement membrane, or by releasing growth factors or other humoral agents (prostaglandins and/or nitric oxide). Immunohistochemistry in stimulated or stretched muscles showed no evidence for expression of mRNA for basic fibroblast growth factor (bFGF), or the growth factor itself, but a low molecular mass endothelial cell-stimulating angiogenic factor (ESAF) (77) was increased in (a), (b), and (d). The involvement of prostaglandins and nitric oxide was demonstrated by the finding of attenuated incorporation of BrdU into capillary-linked nuclei in stimulated muscles after administration of indomethacin or L-NNA. Thus, changes in the microcirculation leading to increased shear stress and/or capillary wall tension may stimulate proliferation of endothelial cells either directly, or by release of various humoral factors. However, extravascular mechanical factors have also to be taken into account.