Mutations at the stoned locus of Drosophila melanogaster produce a reversible temperature-sensitive debilitation. At permissive temperatures they also exhibit an unusual jump response to a light-off stimulus. An increase in the amplitude of the off-transient of the electroretinogram (ERG) is associated with the abnormal jump. Both the jump response and the increased amplitude of the off-transient are shown to be dependent on the duration of the light pulse prior to the light-off stimulus. In stoned flies which are light adapted, the jump response, as measured by recording from the indirect flight muscles, is seen to habituate with increasing light-off frequency. This habituation corresponds to the decrease in the amplitude of the off-transient that also occurs with high-frequency stimulation. Another visual mutant tan, removes the off and on-transients of the ERG. The combining of the stoned mutation with tan in the tan, stoned double mutant results in the loss of the jump behavior as well as the partial restoration of the off-transient to an otherwise tan-like ERG. The relationship between the increase in the amplitude of the off-transient in stoned flies and the eliciting of the jump response is discussed.