Glucose transporter protein levels have been investigated in mdx and control (C57Bl/1O) mice. Crude membrane fractions (microsomes plus plasma membranes) were prepared from skeletal muscle, heart, diaphragm and brain of 5-6-week-old and 6-7-month-old control and mdx mice. Using Western blot analysis with C-terminal-specific anti-peptide antibodies, we investigated the glucose transporters GLUT4 in the different muscle tissues and GLUTI in brain. In skeletal tissue from the hindlegs, GLUT4 was increased by -55 % in mdx mice compared with control mice at both ages studied. In the diaphragm, the amount of GLUT4 protein was unchanged in young mdx mice, and was decreased by 37.4 + 4.7% in older mice compared with agematched control mice. No difference was observed between mdx and control mice in the amounts of GLUT4 and GLUTI in heart and brain preparations respectively. To determine whether the change in GLUT4 protein observed in the diaphragm and