Immunohistochemical techniques were used to describe the presence of endogenous levels of enkephalin-like immunoreactivity (ELI) in the developing central nervous system of the rat up until birth. The appearance and prenatal ontogeny of nerve cell bodies, nerve fiber pathways and varicose terminal plexuses expressing ELI were thus mapped using the indirect fluorescence immunohistochemical technique and fluorescein- and rhodamine-conjugated second antisera. ELI was first observed in beaded fibers along the midline from ventral pons to cervical spinal cord by prenatal day 15. Fluorescence could not be observed in the brains of 14 day old fetuses, although the adrenal medulla did show ELI at this stage. ELI-positive cell bodies at prenatal day 18 were found in most areas in which they have previously been described in adult rats. Many of the cells observed at prenatal day 18 were, however, not seen in untreated full term fetuses with the techniques used here. ELI-positive fibers and terminal fields begin to approach adult distributions at prenatal day 21--22. Several ELI-positive axon pathways which have not been previously reported from adult or developing brain, such as e.g. pathways in medial neocortex, in fasciculus retroflexus, in tractus mammillothalamicus, between ventral hypothalamus and globus pallidus and between ventromedial pons and the locus coeruleus area, are described. Semischematic maps are presented which outline all ELI-positive material as seen in sagittal projections of the central nervous system of 15 day, 18 day and full term fetuses. Maps of representative transverse sections of the full term brain are also included.