Total respiratory resistance and reactance from 3 to 30 Hz were determined by the method of forced random noise oscillation in 12 normal male subjects before and after bronchodilatation and bronchoconstriction induced by deep breaths of aerosols of isoproterenol and atropine and of methacholine and histamine. Isoproterenol and atropine induced small decreases in total respiratory resistance at most frequencies, and isoproterenol decreased resonant frequency slightly (P less than 0.01). After administration of both methacholine and histamine, resonance frequency increased (P less than 0.01) and total respiratory resistance became more frequency-dependent, increasing mainly in the lower frequency range. In six of the subjects, we attempted to produce central deposition of methacholine by rapid, shallow breathing and peripheral deposition of the drug by slow, deep breathing. Only two subjects had suggestive evidence of central bronchoconstriction. No difference was noted, however, in the impedance behavior with either type of breathing. In awake humans, impedance analysis does not seem to distinguish between central and peripheral airway constriction.