Isolated rat liver mitochondria were incubated in the presence of 6 micrograms/ml of Photofrin and irradiated at the wavelength of 365 nm. After 45 s irradiation (30 W/m2), coupling defined as stimulation of respiration by externally added adenosine 5'-diphosphate (ADP) is totally lost. In contrast, membrane potential created by addition of succinate or adenosine 5'-triphosphate (ATP) is only slightly affected. Similarly, the ADP/O ratio is not modified after 20 s irradiation. These data suggest that modification of the mitochondrial membrane potential is not a primary event after irradiation.