Membrane-bound enzyme systems were irradiated with sinusoidally modulated microwave radiation in a spectrophotometric apparatus in which enzyme activity was measured during irradiation. The enzymes chosen for study were cytochrome oxidase, a key enzyme in the electron-transport chain that is located in the inner membrane of mitochondria, and adenosine triphosphatase (ATPase) from red-blood-cell membrane, which is involved in maintaining the sodium-potassium balance of the cell. The enzyme systems were prepared from rat tissue and were subjected to 2450-MHz radiation sinusoidally modulated at 16, 30, 90, and 120 Hz; the specific absorption rate was 26 mW g−1. Temperature was controlled at 25±1.0°C in all experiments. Irradiation began immediately after initiation of the reaction and continued for several minutes while the reaction rate was being monitored. Cytochrome oxidase activity was measured directly by monitoring the change in absorbance of its substrate, cytochrome c, at 550 nm. Activity of ATPase was measured by a coupled-enzyme system in which the rate of conversion of ATP to ADP was followed at 340 nm by stoichiometric oxidation of NADPH. The enzymes pyruvate kinase and lactate dehydrogenase performed the intermediate steps and were always present in excess so that the activity of ATPase was rate controlling. No statistically significant differences in enzyme activities were obtained between irradiated and control samples at any of the modulation frequencies for either enzyme system.