Capillary supply, the proportion of oxidative fibres and blood flow were studied in fast rat muscles (tibialis anterior, TA, and extensor digitorum, EDL) made ischaemic by ligation of the common iliac artery, in chronically stimulated muscles and in ischaemic chronically stimulated muscles. Stimulation was carried out for 6 h/day at 10 Hz (three periods of 2 h with 90-120-rain intervals between stimulations) for 10 -12 days using electrodes implanted in the vicinity of the lateral popliteal nerve. Blood flow (measured by radioactive microspheres) was 3.62 _+ 0.52 ml. 100 g-1 -min-~ at rest and 78.4 _+ 14.6 ml. 100 g-~" rain-~ (mean _+ SEM) during isometric contractions at 4 Hz. Ischaemic muscles had significantly lower blood flow at rest as well as during contractions (72 _+ 14% and 25 _+ 4% of the values in contralateral muscles respectively). Stimulated muscles had significantly higher flow than contralateral control muscles during contractions; stimulted ischaemic muscles had normal blood flow at rest, but the increase in flow during contractions was limited to a similar extent to that in ischaemic muscles alone. Of all anatomically present capillaries (staining for alkaline phosphatase in frozen sections) the capillary/fibre ratio increased by 36% in stimulated tibialis anterior, but was not significantly different from control muscles in stimulated ischaemic TA and was even lower than in control muscles in stimulated ischaemic EDL. The proportion of fast oxidative fibres (estimated on the basis ofhistochemical staining for myosin ATPase and succinate dehydrogenase) increased from 53.2β’ in normal EDL to 82.0___2.3% in chronically stimulated EDL and to 100% in chronically stimulted ischaemic muscles. Chronic stimulation of ischaemic muscles resulted in muscle fibre damage and consequently a loss of muscle weight (34.7 _+ 7.3% in EDL and 45.4 + 4% in TA). Thus capillary growth occurring in chronically stimulated muscles with normal blood flow was prevented when blood supply was limited, although the ischemic stimulated muscles showed a higher oxidative capacity. This finding emphasizes the Offprint requests to." O. Hudlicka role of blood flow in the initiation of capillary proliferation.