The peroxidase‐antiperoxidase (PAP) technique is used to determine the time of first detection and distribution of neuronal perikarya and processes showing substance P (SP)‐ and enkephalin (EN)‐like immunoreactivity (LI) in the central and peripheral nervous system of the fetal rat. SPLI is first detected in central perikarya at E15. By E18, SPLI is less intensely localized in certain nuclei and shows heavier accumulations of immunoreactivity in central processes in peripheral sensory neurons. ENLI is also widely distributed in perikarya and processes of the central nervous system at E18, but is not detected at E15. The distribution of SPLI and ENLI at E18 is comparable to that previously described in the adult brain with the following exceptions: (1) labeled perikarya and processes are not detected in the rostral forebrain; (2) axonal pathways are intensely labeled whereas most terminal fields are devoid of immunoreactivity.
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