Adrenal medulla cells are cholinoceptive cells. Stimulation of the acetylcholine receptor causes the influx of Ca to the cells, and Ca acts as the coupler of the stimulus–secretion coupling. In this study, the authors investigated the effects of halothane on the receptor-mediated influx of45Ca using cultured bovine adrenal medulla cells. Halothane at clinical concentrations (0.5–2%) inhibited the influx of45Ca caused by carbachol, with simultaneous inhibition of catecholamine secretion. The influx of45Ca and the secretion of catecholamines caused by K depolarization were inhibited by a large concentration of Mg, which competes with Ca at Ca channels, but not inhibited by halothane. Inhibition of the45Ca influx by halothane was not overcome by increase in the carbachol concentration. Inhibition of the45Ca influx by halothane was examined in comparison with that caused by a large concentration of Mg by the application of Scatchard analysis as the function of the external Ca concentration. Halothane decreased the maximal influx of45Ca without altering the apparent kinetic constant of Ca to Ca channels. On the contrary, a large concentration of Mg increased the apparent kinetic constant without altering the maximal influx of45Ca. Based on these findings, the authors suggest that inhibition of the45Ca influx by halothane was not due to the direct competitive inhibition of Ca channels, nor to the competitive antagonism of agonist-receptor interaction. As a possibility, halothane seems to inhibit the receptor-mediated activation of Ca channels through the interference of coupling between the receptor and Ca channels.