1. Morphological correlates of circadian changes in eye sensitivity to light measured electrophysiologically were sought in the cockroach, Leucophaea maderae. Cross sections of ommatidia removed at subjective midday and subjective midnight on 3 successive days from roaches held under constant darkness (DD) at 25 +/- 2 degrees C were examined using a transmission electron microscope for morphological differences related to sampling time. 2. The temporal difference in submicrovillar cisternae (SMC) area appeared to exhibit a circadian rhythm, however, the amplitude of this temporal difference measured under DD was less than that observed under LD 12:12 conditions. SMC areas characteristic at nighttime were achieved at subjective midnights but the area diminished only partially toward the daytime state on subjective middays. 3. Rhabdom area remained constant and the daily rhythm of screening pigment granules (SPG) arrangement about the rhabdom was not observed under conditions of constant darkness. 4. Results of this study indicate that a pacemaker(s) actively influences the change in the SMC toward the nighttime state, whereas, the change toward the daytime state results from a passive mechanism that possibly could be accelerated by light.