130 muscle biopsies were taken from the gastrocnemius of 82 patients with different degrees of peripheral vascular disease (PVD) and 19 normal controls for histochemical analysis, and 30 patients and 7 controls for biochemical analysis of "aerobic" and "anaerobic" enzymes. The results showed that the gastrocnemius of patients with PVD did not adapt by increasing its "aerobic" potential as previously suggested. Histochemical studies showed that the cross-sectional area of both Type 1 and Type 2 muscle fibres became smaller than those from age-matched controls in the presence of PVD, and in some cases biopsies from both legs confirmed that this trend was greater in those limbs with increasing evidence of PVD. There is also some evidence to suggest a decrease in absolute capillary numbers per muscle fibre. The biochemical assays confirmed decreased levels of aerobic enzymes with increasing evidence of PVD as judged by ankle systolic pressure, and those patients with intermittent claudication showed some evidence of increased anaerobic enzyme levels in comparison to both the normal controls and those with rest pain. Patients with PVD do not adapt to ischaemia/anoxia by increasing their aerobic capability, but show signs of muscle atrophy probably due to reduced mobility. Training regimes may benefit patients with intermittent claudication by reversing these changes.