Optical recording of transmembrane voltage changes with the use of potentiometric dyes has opened the possibility of determining spatial patterns of electrical activity in excitable tissues. To follow such activation patterns on the cellular/subcellular level in heart cell cultures, a recording system was developed that features both high spatial resolution (4-200 microm) and high temporal resolution (uncertainty in the determination of delays between fast rising signals of +/-1 micros). Central to the system is a fiber optic image conduit consisting of 379 individual optical fibers. At one end the fibers are fused to form an input window that matches the size of the field of view of the microscope. At the other end, the fibers are loose, permitting a selectable subset to be connected to 80 discrete photodetectors. This design allows the sensitive area of the imager to be adapted to regions of interest in a given preparation, thus making optimal use of the limited number of detectors. Furthermore, by using a second fiber optic imager, individual photodetectors can be assigned to different optical ports, thus providing the means for fast and simultaneous dual-emission wavelength measurements. This feature permitted the elimination of motion artifacts arising from the myocytes without the use of contraction-suppressing drugs.