The development of microglial cells in the postnatal rat retina is described using histochemical techniques for acid phosphatase and peroxidase as well as silver impregnations for microglia. On the second postnatal day, round acid phosphatase-positive macrophages appeared on the vitreal surface of retina, locating themselves close to developing blood vessels. Later, microglial precursors invaded retinal tissues, reaching the outer plexiform layer by the tenth postnatal day. In all stages studied, microglia or their precursors were peroxidase-negative. The transformation of round microglial precursors into adult ramified microglia is also described. Owing to the relation found between developing microglia and blood vessels, a vascular origin is proposed for the retinal microglial cells.