Supramaximal electrical field stimulation was employed to produce biphasic response in segments of cervical and thoracic tracheas and in first generation of bronchi isolated from guinea pigs. This response consists of contraction (as measured by maximal active tension, ATmax) followed by relaxation (as measured by maximal active relaxation, ARmax). First, the effect of electrical stimulation was studied in relation to frequency and duration of impulse. Highest degree of relaxation (ARmax) was achieved with a frequency of stimulus of 40 pulses per sec for segments of trachea, and 60 pulses per sec for first generation bronchi, and with duration of pulse of 1.0 msec for segments of trachea, and 2.0-2.5 msec for bronchi, and with duration of pulse of 1.0 msec for segments of trachea, and 2.0-2.5 msec for bronchi. Secondly, mean length-tension relationship was determined. It was found that maximal relaxation (ARmax) occurred at 160 +/- 5% of Lmax in case of tracheal segments and 160 +/- 2% of Lmax for bronchial preparations. Using propranolol pretreatment (10(-4) M), which inhibited participation of adrenergic receptors in relaxation, it was determined that approximately 50% relaxation of airway smooth muscle of the normal guinea pig is mediated through the purinergic nervous system. Both tracheas and bronchi isolated from sensitized guinea pigs, chronically exposed to antigen, showed a significant decrease in overall relaxation (P less than 0.01). However, the per cent decrease in relaxation after propranolol was not different in sensitized animals than that found in controls. The data suggest that relaxation of segments of airways isolated from animals chronically exposed to antigen is decreased in consequence of alterations in both purinergic and adrenergic nervous systems.