Exposure of rabbit retina to cytochalasin D (CD) via a single intraocular injection results in basal rod outer segment (ROS) and cone OS (COS) discs with abnormally large diameters; the overgrown OS membranes extend along either the cell outer or inner segment. Twenty-four hours after the injection, basal ROS and COS discs appear to have recovered their normal diameter, indicating the reversibility of this drug's effect. These data support both the evagination hypothesis for disc morphogenesis and the hypothesis that f-actin's role at the ROS base is to regulate the initiation of membrane evagination and disc diameter.