The effect of 1 mg inhaled prazosin on bronchoconstriction induced by methoxamine was investigated in seven asthmatic subjects. Prazosin caused significant inhibition of the methoxamine-induced bronchoconstriction in six of the seven patients. These findings suggest that methoxamine produces bronchoconstriction in asthmatic subjects via stimulation of alpha-adrenoceptors. In previous studies propylene glycol has been used as a vehicle for delivery of prazosin. This substance was found to cause significant inhibition of methoxamine effects and to shift the dose response curve to histamine to the right in four of seven patients.