Electrical responses to light stimuli were recorded intracellularly from the retinula cells and the laminar cells of the wild type and the x-7 mutant of Drosophila melanogaster in order to determine the cellular origin of the on-transient of the insect electroretinogram. The response of the retinula cell of the mutant x-7 closely resembles that of the wild type even though the ontranstent is absent from the electroretinogram of x-7. Neither contains a spike-like component that can be identified with the on-transient of the electroretinogram. However, a spike-like response resembling the on-transient in latency and time course has been obtained from one of the several types of cells in the lamina of the wild type. Moreover, the same response could not be obtained from the laminar cells of the mutant x-7. Thus, the on-transient does not appear to originate from the retinula cell but probably from one or more of several types of cells in the lamina.