The development and cellular origin of chondroitin sulfate-type proteoglycans in the interphotoreceptor matrix (IPM) of the mouse retina were examined with electron microscopy after staining with the cationic dye, Cupromeronic Blue. In the IPM of the developing retina, three structural types of Cupromeronic Blue-positive filaments were observed. Type A filaments measured 45-55 nm in length and around 5 nm in diameter. At birth, a few type A filaments were present in the IPM and they increased in number during subsequent days postpartum. Type A filaments were present in high density in the cytoplasm of the pigment epithelial cells at all stages examined where they were associated with the Golgi apparatus and cytoplasmic vesicles. Type B filaments, measuring 60-80 nm in length and 5-10 nm in diameter, were occasionally observed in the IPM of the newborn mouse. Type B filaments increased in number during subsequent development and by 14 days postpartum, their location was restricted predominantly to the IPM around photoreceptor inner segments and the connecting cilia. Type C filaments, measuring 15-25 nm diameter with extremely long profiles (up to several microns), were first consistently observed in the IPM at 6 days postpartum, the time when photoreceptor outer segments begin to develop. During subsequent days postpartum, type C filaments increased in number. By 18 days postpartum, when photoreceptors have achieved their adult length, type C filaments were interconnected in a net-like meshwork investing the photoreceptor outer segments. All three Cupromeronic Blue-positive filament types were sensitive to chondroitinase AC, but not to Streptomyces hyaluronidase. Cupromeronic Blue-stained filaments were rarely observed in the cytoplasm of the photoreceptor or Müller cells at any of the stages examined. These findings suggest that proteoglycans of the chondroitin sulfate-type present between the retina and pigment epithelium of the adult mouse are secreted principally by the retinal pigment epithelial cells, and that these components begin to be elaborated into the IPM a few days prior to the stage when photoreceptor outer segment development is initiated. Organization of the proteoglycans to form an extracellular meshwork surrounding the photoreceptors takes place during the period when photoreceptor outer segments are elongating to achieve their adult length.