Two types of monopolar cell interneurons, each with a single representative in every unit cartridge of the first optic neuropil, or lamina, of the housefly's optic lobe, have axons that undergo cyclical changes in diameter. The axons are largest during the beginning of day in a normal LD light cycle and smallest during the middle of the night, changes that were however significant only for one of the cells (L2). The axon cross-sectional area and its cyclical change for both L1 and L2 were both larger in the proximal lamina. The changes are not a simple consequence of relative osmotic change. Dehydration paradoxically increases axon size, and also fails to alter the day/night rhythm of axon size changes. Under conditions of constant darkness, both axons decrease in size, and one of the cells (L2) retains its cyclical size changes, being larger in the subjective day than in the subjective night. Under conditions of constant light, both axons increase in size, and L2 again shows a cyclical size change, just as under conditions of constant darkness. These changes seen under constant conditions are, by definition, circadian in origin. The effects of exposure to light or darkness can partially reset these circadian changes. One extra hour of light during the day increases the size of L1 and L2, whereas 1 hr of extra dark during the night does not decrease their size. It takes 13 hr of light to reverse the rhythm in size. The mechanism for all these changes is unclear but may involve ionic fluxes, possibly that are secondary to osmotic shifts and probably that involve at least two independent processes.