The uptake of alpha-aminoisobutyric acid (AIB) was studied in soleus muscles isolated from lean and gold thioglucose-lesioned obese mice (GTG-obese), both in basal and in insulin-stimulated conditions. The basal (i.e., non-insulin-stimulated) influx of AIB was decreased in muscles of GTG-obese mice because of a decrease in the apparent Vmax of the transport system. In muscles from both types of mice, insulin stimulated AIB influx by increasing the apparent Vmax of transport; this effect was partially dependent on new protein synthesis. In muscles of GTG-obese mice, the effect of insulin was altered in two ways: the absolute value of the maximally insulin-stimulated AIB influx was lower than in muscles of lean controls, and the dose response of insulin-stimulated AIB influx was shifted to the right (half-maximally effective concentration, EC50 congruent to 1.7 nM) compared to that observed in muscles of lean animals (EC50 congruent to 0.7 nM). It is concluded that in muscles of obese mice a) amino acid transport per se is altered, and b) the sensitivity of this process to the stimulatory effect of insulin is markedly diminished.