Optical methods using changes in fluorescence and absorption of voltage-sensitive dyes were developed to record electrical activity from processes of nerve cells grown in monolayer culture. For transmission measurements, a merocyanine dye was discovered but was more sensitive than others previously tested on cultured neurons. Action potentials from the somata of these cells were detected without averaging, with a signal-to-noise ratio of 20:1. With this dye, electrical responses were simultaneously recorded from many points along the arborization of neuroblastoma cells by using a 10 X 10 array of photodiodes positioned in the microscope image plane. Frequently different processes had different shapes of electrical responses, suggesting regional specializations. Fluorescence measurements with an oxonol dye proved to be more sensitive than transmission measurements, particularly when recording from small processes. By changing the position of the cell relative to a laser microbeam while recording electrically from the cell body, it was possible to monitor the membrane potential in the cell body and in the process simultaneously. From the delay in response in the process, a lower limit for the mean conduction velocity of 0.2-0.6 m/sec was found for 2- to 6- micrometers processes. The mean space constants of processes were estimated by comparing the amplitudes of passive voltage responses in the cell body and growth cone. A lower limit of 400-950 micrometers was obtained for 4- to 7-micrometers processes.