Liver mitochondria were exposed in vitro at 30 degrees C to microwave radiation (2.45 GHz) during the following states of respiration: resting, state 1; substrate dependent, state 2; ADP stimulated, state 3; and ADP depleted, state 4. At 10 or 100 mW/g, with succinate as substrate, no effect of exposure was observed on states 1-4 or the respiratory control index (state 3/state 4) of either tightly or loosely coupled mitochondria. When glutamate was used as substrate, no effects were observed at 10 mW/g. However, in the loosely coupled mitochondria the 100 mW/g exposure produced an increase in states 2 and 4 and a decrease in the respiratory control index. The results suggest that the function of loosely coupled mitochondria can be affected at high power levels of microwave radiation.