We investigated the inhibitory effects of nicardipine on airway smooth muscle constriction in the guinea pig via both in vitro and in vivo preparations. In the in vitro studies, we created dose-response curves of the excised tracheal strip. The increase in respiratory overflow produced by leukotriene (LT) D4 was measured in the in vivo preparation. In the organ bath, nicardipine (1-12 micrograms/ml) significantly inhibited constriction of the isolated tracheal spiral strip induced by 7 different agonists, acetylcholine (Ach), histamine (HA), serotonin (5-HT), prostaglandin F2 alpha (PGF2 alpha), slow-reacting substance of anaphylaxis (SRS-A), and LTC4 and LTD4. Compared to nifedipine and verapamil (data in the literature), the inhibitory potency of nicardipine on the constriction of the isolated guinea pig tracheal strip is 100-1,000 times greater. In the in vivo studies, nicardipine (50 and 100 micrograms/kg) significantly attenuated the LTD4-induced increase in respiratory overflow. These results suggest that nicardipine inhibits bronchoconstrictor-induced constriction of airway smooth muscle both in vitro and in vivo, and that it is a potent bronchodilator. The inhibitory characteristics of nicardipine may prove to be applicable clinically.