Mechanisms underlying cyclic motor pattern generation by the lobster stomatogastric ganglion are being studied using a new method of inactivating single cells. This method consists of filling the cell to be inactivated with the fluorescent dye Lucifer Yellow iontophoretically and irradiating it for a few minutes with intense blue light. The cell's death does not affect the physiological properties of the remaining cells in the ganglion. The role each cell plays in the pattern-generating mechanism can be ascertained by observing the effect of its removal on the motor output pattern.