Although calcium antagonists such as verapamil are used primarily in cardiovascular disease, they appear to relax smooth muscle generally. Therefore, the possibility that verapamil might have bronchodilator properties was explored using the guinea-pig tracheal ring technique. Verapamil was found to produce considerable tracheal smooth muscle relaxation from a threshold concentration of 2 X 10(-7) M and with maximum effect at 10(-3) M. The responses to the contractile agonists histamine and prostaglandin F2 alpha and especially methacholine and serotonin were substantially reduced by prior administration of verapamil. Verapamil 2 X 10(-4) M was equally effective as isoprenaline 10(-8) M in producing 50% maximum direct relaxation but was more effective than isoprenaline as an antagonist of the contractile agonists, methacholine, histamine and serotonin, but not prostaglandin F2 alpha. Verapamil abolished the contractile responses to barium chloride. It is concluded that, although verapamil was not very potent as a direct bronchodilator, it could potentially be of prophylactic benefit in asthma because of its efficacy as an antagonist of common contractile agonists.