Spontaneous contractions of smooth muscle were observed during microwave irradiation. Isolated gut segments of adult male albino rats were exposed to 1-GHz, continuous-wave radiation by means of a capacitive-plate exposure system. For stabilization, the gut segment was maintained initially at 7°C for one hour and then at 36°C for one hour in a modified Ringer's solution prior to irradiation. During recording of contractions the gut segment was suspended in a 10-ml plastic tube and was bathed by Ringer's solution (pH 7.5) at 36°C at a flow rate of 6 ml min−1 from a 500-ml reservoir. Contractions were measured by a strain gauge. Frequency distributions of 500 contractions each were made before and during a single exposure and were displayed as interval-histogram patterns. Four gut preparations were exposed to radiation at intensities that resulted in specific absorption rates of either 1.2, 2.3, or 6.9 mW g−1. For each absorption rate the response patterns for the four gut preparations representing a before-exposure (control) condition were summed and compared with the summed response patterns of the same preparations during exposure. No effect of irradiation by electromagnetic energy on the rate of spontaneous contractions of smooth muscle was observed.