1. Insulin and adaptive regulation are known to stimulate system A amino acid transport activity in skeletal muscle. The present study was designed to investigate whether activation of system A in muscle is a consequence of processes which rely on microtubule or microfilament function. To that end, extensor digitorum longus (EDL) muscles were incubated in the presence of colchicine and cytochalasin D, well-known inhibitors of microtubule and microfilament activity respectively. 2. Basal alpha-(methyl)aminoisobutyric acid (MeAIB) uptake decreased after incubation with 5 microM-colchicine in a time-dependent manner. In keeping with this, adaptive regulation of MeAIB uptake caused by prolonged incubation in the absence of amino acids was substantially decreased in the presence of colchicine. 3. Under these conditions, stimulation of MeAIB uptake by insulin was unaltered in muscle in the presence of colchicine. This contrasted with the insulin-induced stimulation of MeAIB uptake by isolated rat hepatocytes, which was markedly decreased by colchicine. 4. Cytochalasin D, an agent that disrupts microfilaments, did not inhibit basal or insulin-stimulated MeAIB uptake by the incubated muscle. 5. Neither colchicine nor cytochalasin D modified the stimulatory effect of insulin on 3-O-methylglucose uptake by EDL muscle. 6. We conclude that up-regulation of system A by synthesis of new carriers depends on the integrity of microtubular function both in skeletal muscle and in hepatocytes. Microtubules might play a role in the movement of system A-containing vesicles from the Golgi network to the plasma membrane.