Irradiation (960-MHz) of isolated post-pyloric segments of rat gut at a specific absorption rate (SAR) of 1.5–5.5 W/kg caused an increase in peristaltic activity. A pressure transducer was used to monitor waveforms of muscle movements in a canulated section of intestine. The preparation was suspended in isothermal, aerated, modified Ringer's solution. Microwave radiation was applied with a parallel-plate capacitor-type radiator. The SAR was calculated from temperature measurements made with a liquid-crystal optical-fiber (LCOF) temperature probe. As reported previously, similar radiation applied to isolated turtle and rat heart caused bradycardia. These effects, in both the heart and gut, are thought to be neurally mediated by the release of transmitter substances by the autonomic nervous system (ANS). Generally, these effects can be modified by the addition of drugs that are effective at synapses within the ANS. In both the heart and gut, sympathetic and parasympathetic effects can be separated by the application of drugs that selectively block one or the other of the two divisions of the ANS.