Polarized light microscopy reveals that the structural parameters of Rana pipiens rod outer segments (ROS) are not uniform along the cell axis. In addition to a pronounced birefringence (delta n) gradient found in the basal half of most ROS, periodic delta n bands are seen in approximately 10% of intact ROS isolated by agitating retinas in frog Ringer's solution. These small delta n differences appear as very faint light and dark striations that have a period and width that depends on the duration of light and dark exposure. In ROS from frogs kept on a 14 hr light/10 hr dark cycle at 20 degrees to 22.5 degrees C, the band period for a light-dark band pair is 1.0 to 1.6 micron. Portions of ROS produced during total darkness or constant light are free of distinct periodic bands. Quantitative delta n measurements show that the ROS sections generated in the dark have a relatively higher delta n than those produced in light. Band contrast is irreversibly enhanced when ROS are treated with the calcium ionophore A23187 in the presence of calcium-free saline solution. These results indicate that the synthesis of some calcium-sensitive ROS component is different when the frog is in the dark than when exposed to light.