effect of inhaled histamine on occlusion pressure and breathing pattern in asthmatic patients in animals histamine inhalation is known to increase either respiratory frequency or respiratory drive by stimulation of airway vagal sensitive endings however it is not well known whether these changes are concomitant in man in order to elucidate this point we carried out the present investigation in thirtyfive asthmatic patients who underwent bronchial provocation test by progressively doubling the dose of inhaled histamine bronchial reactivity to histamine allowed two populations of patients to be defined group i with moderate and group ii with mild increased reactivity in the twentythree group i patients neuromuscular inspiratory drive assessed by mouth occlusion pressure p01 was found to be significantly increased while no significant changes in breathing pattern were noted in the twelve group ii patients histamine did not modify p01 or breathing pattern however we were able to separate in group i a subgroup of ten patients as with atopic asthma in which histamineinduced increase in p01 was paralleled by rapid and shallow breathing rsb changes in p01 and breathing pattern did not depend on baseline airway calibre in group i after bronchoconstriction had been reversed by inhaling a beta 2agonist bronchodilator agent fenoterol p01 decreased significantly and rsb was found to be reversed however these changes were not interrelated we concluded that in asthmatics histamineinduced increase in p01 is not necessarily paralleled by nor related with change in breathing pattern and in atopics a sensitization of vagal receptors could account for the concomitance of enhanced p01 with rsb