Determining the birthdate of neurons and glia in relation lO the time period of neurochemical 1.nd cellular differentiation are necessary for understanding early developmental events in the central nerv.>us system (CNS). Bromodeoxyuridine (BrdU). a thymidine analog has recently become an alternative to the tritiated thymidine autoradiography for the study of cytogenesis in the CNS. Our laboratory utilizes the Brazilian opossum, Monodelphis domestica to study the ontogeny of neuropeptide systems and the development of the visual system in the mammalian brain. Opossum pups are born in an extremely immature state before neurogenesis and morphogenesis are completed and provide an excellent experimental model for developmental studies. To this end, we have developed single, double. and triple label BrctU immunohistochemical procedures for simultaneous study of cytogenesis and cellular differentiation in the opossum brain and retina. These methods are simple, rapid, and very effective and could be widely applied for understanding developmental events in the CNS.