Royal College of Surgeons (RCS) rats exhibit an hereditary defect in phagocytosis of the tips of the photoreceptor cell rod outer segments (ROS) which leads to degeneration of the retinal visual cells. The lipid composition of outer segment membranes of these rats was analysed and compared to those of normal rats to determine whether there are differences between the normal and mutant rat ROS. The cholesterol distribution in ROS disk membranes from normal and RCS rats was investigated using a digitonin induced change in membrane density. Normal rat ROS disks varied in cholesterol to phospholipid mole ratio from 0.36 to 0.03. The disk membranes from RCS rats, however, do not exhibit the same marked cholesterol heterogeneity. The mean molar ratio of cholesterol to phospholipid in the disk membranes of normal rats is 0.11 while that found in the RCS rats is 0.14. The ROS plasma membrane of dystrophic rats also has a lower cholesterol to phospholipid ratio (0.20) than is found in the normal rat (0.40). The phospholipid headgroup composition of RCS disks and plasma membrane were determined. RCS disks were shown to differ from those of normal animals. The cholesterol content of ROS disks may be governed by the phospholipid composition.