Whilst it is well known that exercise can cause angiogenesis in skeletal muscle low intensity or short duration activity does not cause widespread capillary proliferation, implying that an activation threshold must be overcome for angiogenesis to occur. In vitro experiments suggest that specific concentrations of growth factors are needed to produce responses consistent with angiogenesis (Xue & Greisler, 2002), but it is not clear whether these fall inside a physiological range, or if this is indeed the mechanism through which control is exerted. We therefore examined three models of angiogenesis that show a graded response in muscle overload and subsequent angiogenesis. The present study examined whether there was a graded response in vascular endothelial growth factor (VEGF), or its main functional receptor, Flk-1, as VEGF is essential for overload-induced angiogenesis (Williams et al. 2004). Male Sprague-Dawley rats were anaesthetised with 2% fluothane in oxygen and subjected to either extirpation of the m. tibialis anterior (TA), tenotomy of the main tendon of the TA or ligotomy of the extensor retinaculum, the ligament that holds the TA in place. Animals were randomised with respect to side, and treated postoperatively with analgesics and antibiotics (Temgesic, Duplocillin). These interventions cause a reducing severity of overload of the m. extensor digitorum longus (Badr et al. 2003), which was studied at 3, 7, 14 and 28 days after surgery. Western blots of VEGF and Flk-1, with VEGF ELISA, showed no significant difference in time course or magnitude of response with the degree of overload. These data suggest that the angiogenic response to graded levels of muscle overload is mediated by a threshold, rather than a graded increase in VEGF and Flk-1. The degree of angiogenesis observed is therefore probably controlled by interactions with other pro-angiogenic stimuli, rather than by a graded response in the primary growth factor alone.