In the present study we examined mRNA and protein levels for the muscle/adipose tissue glucose transporter (GLUT-4) in various tissues of spontaneously obese mice (C57BL/KsJ, db/db) and their lean littermates (db/+). Obese (db/db) mice were studied at 5 wk of age, when they were rapidly gaining weight and were severely insulin resistant, evidenced by hyperglycemia (plasma glucose 683 +/- 60 vs. 169 +/- 4 mg/dl in db/+, P less than 0.05) and hyperinsulinemia (plasma insulin 14.9 +/- 0.53 vs. 1.52 +/- 0.08 ng/ml in db/+, P less than 0.05). The GLUT-4 mRNA was reduced in quadriceps muscle (67.5 +/- 8.5%, P = 0.02), but unaltered in adipose tissue (120 +/- 19%, NS), heart (95.7 +/- 6.1%, NS), or diaphragm (75.2 +/- 12.1%, NS) in obese (db/db) mice relative to levels in lean littermates. The GLUT-4 protein, measured by quantitative immunoblot analysis using two different GLUT-4 specific antibodies, was not different in five insulin-sensitive tissues including diaphragm, heart, red and white quadriceps muscle, and adipose tissue of obese (db/db) mice compared with tissue levels in lean littermates; these findings were consistent when measured relative to tissue DNA levels as an index of cell number. These data suggest that the marked defect in glucose utilization previously described in skeletal muscle of these young obese mice is not due to a decrease in the level of the major muscle glucose transporter. An alternate step in insulin-dependent activation of the glucose transport process is probably involved.