A computer-controlled 100-channel High Speed Diode Array Spectrophotometer has been developed by the Biomedical Engineering and Instrumentation Program of the National Center for Research Resources and CSL for the Laboratory of Cell Biology (L:CB), NHLBI. It will be used to obtain more complete spectral information about the rapid changes of the reduction and oxidation centers within the protein enzyme cytochrome oxidase. This enzyme is involved in cellular respiration and is located within the inner lipid bilayer of the mitochondrion. The electronic hardware consists of two 48-element photodiode arrays, with each element connected to a discrete A/D converter and, subsequently, to local storage channels. Each channel is capable of acquiring data at 10 microsecond intervals. A fast personal computer (PC) is used to control the spectrophotometer. Both timing control signals transmitted from the PC and the received data from the A/D channels are conveyed through a 40-bit parallel interface. The Computer Systems Laboratory assisted in developing the PC interface to the spectrophotometer and in creating the data acquisition and control software. It is anticipated that the commercial data manipulation language MLAB will be used for analysis of the data. The spectrophotometer has been built and delivered to NHLBI. Laboratory testing of both the hardware and the software systems has been completed.