An apparatus for accurately timing the sequence of spectroscopic events in turbid suspensions of the cytochrome oxidase system has been developed. This apparatus measures simultaneously small optical density changes occurring at two wavelengths in the visible region with a small spectral interval in spite of the large scattering of light by the heart muscle suspensions. The cross talk between the two channels corresponds to an optical density change of less than 5×10−4. Transients arising from the modulation and demodulation processes cause no difficulty. The error of optical measurements caused by the noise level in the photocurrent is 2×10−4for a rise time (10–90 percent) of 1 sec. Some typical experimental results are included.