An apparatus for simultaneous measurement of 90° light scattering, pyridine nucleotide fluorescence and oxygen consumption of mitochondria was designed and constructed for the purpose to study the mitochondrial structure and function. Oxygen consumption was measured by rotating platinum eleotrode by the modification of Hagihara's method, attached in the cell of the apparatus. Mitochondrial swelling and shrinkage were measured by 90° light scattering at 650mμ. The monochrome light was made by plism monochromator and was led to the cell of the apparatus. Scattered light of 650mμ at 90° was filtered through the filter trasmitting 650mμ, excluding visual and ultra-violet radiation under 600mμ. Then the scattered light was registered by photomultiplier tube 1P22 which is a good choice for measurement of the light near red end of spectrum. Relative reduced pyridine nucleotide concentration was measured by fluorometry. Fluorometer was constructed as follow: For excitation, a bright light at 365mμ line of marcury lamp was isolated from other bright light by passing through the Hitachi interference filter (365mμ) or Corning No. 7-54 (9863) which transmits ultraviolet light and excludes visual radiation above 410mμ and was led to the cell by half mirror at a position of light path between the monochromator for 90° light scattering and the cell. The fluorescence light was passed through the filter of Corning No. 3-73 (3389) which transmits visual radiation at approximately 440mμ. Then the fluorescence intensity in the spectral interval set by the grating monochromator was registered by photomultiplier 1P21 which has good signal-to-noise ratio, and is suitable for measurements of compounds that fluoresence between 350 and 650mμ. The scattered light at 650mμ was not affected by excitation light and fluorescence light, and fluorescence intensity was not by scattered light at 650mμ. The simultaneous measurements of the oxidation-reduction of p ridine nucleotides, the respiration states and the change of 90° light scattering is given as an example of the performance of the present apparatus.