Glass microelectrodes were used to study electrophysiological properties of guinea-pig airway smooth muscle (m. trachealis transversus). The resting membrane potential (Em) of airway smooth muscle was found to be -40.4 +/- 0.5 mV (307 cells, 28 preparations). Twenty-seven percent of all cells successfully impaled showed regular spontaneous electrical activity (amplitude of 2-20 mV, with maximum rate of depolarization 15.0 +/- 2.2 mV . sec-1). Forty-four percent of cells showed irregular fluctuations in Em and the remaining cells showed no electrical activity. All three groups of cells had a similar distribution of individual Em values. The sensitization of animals (14 days incubation period) caused a slight but significant increase (P less than 0.001) in Em to -43.1 +/- 0.9 mV. Repeated daily exposure of sensitized animals to aerosolized albumin for two weeks caused a significant reduction of Em to -27.8 +/- 0.8 mV (P less than 0.001). Five weeks repeated exposure caused a further reduction in Em of airway smooth muscle cells to -22.6 +/- 0.7 mV (P less than 0.001). The responses to both histamine (10(-4) M) and isoprenaline (5 x 10(-6) M), as gauged by changes in Em, were altered in the trachea of chronically exposed guinea-pigs. The changes in airway smooth muscle electrical properties were related to the number of times the animals were exposed to inhaled antigen. Even after two weeks of daily exposure, the changes were marked. Airway smooth muscle alteration may be important in the pathogenesis of bronchial asthma.