The crude extract of magnolia bark, an herbal drug, inhibited the secretion of catecholamines from bovine adrenal chromaffin cells stimulated by acetylcholine (ACh) in a concentration-dependent manner (200-900 microg/mL). The extract also diminished the secretion induced by high K(+), which is a stimulus directly depolarizing the plasma membranes, but its inhibition was weaker than that of ACh-evoked secretion. beta-Eudesmol, honokiol, magnolol, and bornyl acetate, but not alpha- and beta-pinenes, all of which are ingredients of magnolia bark, greatly reduced ACh-evoked secretion. beta-Eudesmol and magnolol also inhibited high K(+)-induced secretion to an extent similar to that of ACh-evoked secretion. However, honokiol and bornyl acetate inhibited the secretion induced by high K(+) much less than the secretion evoked by ACh. ACh-induced Na(+) influx and ACh- or high K(+)-induced Ca(2+) influx into the cells were diminished by beta-eudesmol or honokiol. These results indicate that magnolia bark contains some effective components inhibiting the secretion of catecholamines from bovine adrenal chromaffin cells stimulated by ACh due to the antagonism of Na(+) and Ca(2+) influxes into the cells. However, inhibition by the extract of magnolia bark seems to be attributable to honokiol and bornyl acetate. Furthermore, the results indicate that the inhibitory effect of magnolia bark may be associated with its pharmacological effect on activities of the nervous system.