The effects of light adaptation on temporal characteristics of the electroretinogram were studied in two species of diurnal insects. Latency and time to reach peak amplitude (T m ) of the initial deflection of the ERG were the temporal measures used. The experimental regime consisted of repeatedly light-adapting the eye with a conditioning stimulus (S1 and determining the effects of eachS1 on the response to a test stimulus (S2). The compound eyes of the flesh flySarcophaga bullata, and the milkweed bug,Oncopeltus fasciatus, were compared since they differ substantially in ERG waveform and latency of response.