1. In peripheral arterial insufficiency, leg blood flow during exercise is reduced. The aim of this study was to investigate the metabolic response in different muscle types during exercise at reduced versus normal exercise blood flow. 2. A modified rat hindlimb perfusion model was used. Muscle metabolites and distribution of labelled microspheres were analysed in the soleus and the gastrocnemius muscles during exercise induced by sciatic nerve stimulation. 3. Blood flow distribution between the soleus and the gastrocnemius muscles (per unit weight) was 1.7:1 at rest, and this ratio did not change significantly during exercise at reduced flow. 4. There was a more pronounced decrease in the [phosphocreatine], the [glycogen] and the [ATP]/[ADP] ratio as well as a more pronounced increase in the [lactate] and the [lactate]/[Pyruvate] ratio in the gastrocnemius muscle during exercise at reduced blood flow as compared with values obtained at normal exercise flow. In the soleus muscle the difference between the two conditions was confined to an increased [lactate]/[pyruvate] ratio. 5. The results show that a muscle composed mainly of fast-twitch fibres with a high glycolytic and low oxidative capacity is much more susceptible to a reduced exercise flow than a muscle composed of slow-twitch, oxidative fibres. It is suggested that claudicating pain is related to these metabolic changes and it is concluded that pain most probably originates in type II fibers.