1. Capillary density is greater in skeletal muscles comprised of predominantly oxidative (type I) fibres than in those comprised of mainly glycolytic (type II) fibres. In order to investigate further the angiogenic mechanisms involved in muscle capillarization, endothelial-cell-stimulating angiogenic factor activities in various rodent skeletal muscles were compared. 2. Eleven untrained adult male Wistar rats were killed and the predominantly oxidative (type I) muscle,s soleus and heart, the predominantly glycolytic (type II) muscle, extensor digitorum longus, and the mixed-fibre muscle, gastrocnemius, were removed. Each sample was separately homogenized and centrifuged and the supernatants were diafiltered to isolate the low-molecular-mass fraction containing endothelial-cell-stimulating angiogenic activity. This was assayed for its ability to activate latent collagenase and was expressed as units, where 1 unit represents the percentage activation of the enzyme h-1 (mg of protein in the supernatant)-1. 3. The results (medians and ranges) demonstrated significantly greater endothelial-cell-stimulating angiogenic factor activity in extensor digitorum longus muscle (2.14 units, 0.62-2.87 units, n = 13) than in soleus (0.82 units, 0.59-1.79 units, n = 15), gastrocnemius (0.34 units, 0.28-0.40 units, n = 4) or heart (0.43 units, 0.16-0.52 units, n = 11) (P less than 0.01 for each) muscle. 4. These findings suggest that endothelial-cell-stimulating angiogenic activity in muscle is either inversely or not related to the local capillary density, which may be at or near a maximum in physiologically contracting, predominantly oxidative muscles.