Muscle blood flow (MBF) was measured using the local (133)Xenon injection method in patients with Duchenne type muscular dystrophy (six), limb-girdle dystrophy (four), polymyositis (seven), and in normal controls (11). Measurements were made at rest and during hyperaemia induced by ischaemic exercise and by histamine injection. Capillary diffusion capacity was measured with the (51)Cr-EDTA method in three patients with Duchenne dystrophy and in six normal controls. Patients with Duchenne dystrophy had normal MBF during rest and during hyperaemia after exercise and significantly increased MBF after histamine injection. The capillary diffusion capacity was higher than normal, but not significantly. In limb-girdle dystrophy MBF was normal, while in polymyositis it was significantly decreased in all three circulatory states. The relevance of the findings to the proposed ischaemic aetiology of Duchenne dystrophy is considered.