The effect on the microcirculaton of long term administration of the α1 blocker prazosin, which increases peripheral blood flow, was studied in skeletal muscles with differing metabolic profiles. Prazosin (50 mg·litre−1), given ad libitum for 5 weeks in the drinking water, resulted in an increase in capillarity in both fast glycolytic tibialis anterior cortex and slow oxidative soleus muscles of the rat. In tibialis anterior, the velocity of red blood cells through capillaries was increased, a smaller proportion of capillaries had intermittent flow, less time was spent stationary by red blood cells, and hence the calculated volume flow per capillary was increased. In soleus, capillary diameters were larger, but velocity, intermittency and calculated volume flow were unchanged. Therefore, suggested mechanisms for capillary growth induced by prazosin are different for the two muscles studied: increased shear stress and blood/endothelial cell interaction in the case of tibialis anterior, and increased capillary wall tension in the soleus.