Maximum expiratory flow rate at 30 percent of vital capacity above residual volume served as an index of airway obstruction in comparing the effects of leukotriene C and histamine administered by aerosol to five normal persons. Leukotriene C was 600 to 9500 times more potent than histamine on a molar basis in producing an equivalent decrement in the maximum expiratory flow rate at 30 percent of vital capacity above residual volume. The leukotriene C response was slow in onset and prolonged, reminiscent of the effects of aerosol allergen challenge in asthmatic allergic subjects.