We have studied the effects of halothane, a popular volatile anesthetic, on Na(+)-coupled L-alanine cotransporters using rat bone marrow megakaryocytes subjected to patch-clamp whole-cell recordings. L-Alanine applied externally induced an immediate current response which was abolished by the elimination of external NaCl. The optical isomer, D-alanine, induced no such response. A ligand probe for the Na(+)-dependent amino-acid cotransporter (A-system), 2-methylaminoisobutyrate, induced a response comparable to that of L-alanine. The response was fitted by a Michaelis-Menten equation with a Km of 2.5 and 58.5 mM for L-alanine (with 120 mM Na+) and external Na+ (with 10 mM L-alanine), respectively. The volatile anesthetic, halothane, acutely and reversibly inhibited the inward current responsible for the Na(+)-coupled L-alanine cotransporter. Complete inhibition was attained at 1 mM, and 50%-inhibition at 0.35 mM. Halothane in a clinically relevant concentration reduced nutrient uptake via the cotransporter A-system. The sensitivity of the A-system to halothane is comparable to or greater than that of Na+/K+/Cl- cotransporter, sodium/proton exchange, stimulus-secretion coupling, and of many ion channels in nerve cells.