The pharmacological action of mazindol (5-hydroxy-5-p-chlorophenyl-2,3-dihydro-5-imidazo[2,1-a]isoindo l) was examined by studying its effect on glucose uptake by rat tissues using radiolabelled 2-deoxy-D-glucose. The following results were obtained. (1) The rate constant (Ki) of net tissue 2-deoxy-D-glucose uptake increased in the cerebral cortex (4.7-fold, P < 0.05), the hypothalamus (4.6-fold, P < 0.05), heart (4.0 fold, P < 0.05) and skeletal muscles (gastrocnemius, 5.7-fold, P < 0.01; soleus, 4.7-fold, P < 0.05), but not in epididymal adipose tissue in vivo 90 min after intragastric administration of mazindol (20 mg/kg). (2) This increase in Ki values of net tissue 2-deoxy-D-glucose uptake was not observed after addition of mazindol to the diet (40 mg/100 g of diet) for 4 days. (3) Mazindol (16.7 ng/ml to 16.7 micrograms/ml) stimulated 2-deoxy-D-glucose transport into sarcolemmal vesicles of the gastrocnemius muscle in vitro (1.6-1.8-fold, P < 0.05) and this stimulation was blocked by cytochalasin B (10 microM). These findings suggest that mazindol stimulates glucose transport into skeletal muscles by acting on glucose transporters in the sarcolemmal membrane, and suggest that mazindol may stimulate glucose transport into the brain and heart by a similar mechanism.