Galanin is a highly conserved, 29-30 amino acid long putative neuropepide widely distributed throughout the central and peripheral nervous system of several adult species. This protein has been shown to affect the secretion of several hormones and the proliferation of some small cell lung cancer and lactotroph cell lines. Galanin is also expre sed by certain types of neuroendocrine tumors. These findings suggest that galanin could be a mitogen. Mitogens cause cell proliferation in the normal development of the embryo for growth of tissues. Therefore, galanin may be expressed during development of the embryo. The purpose of this study was to determine the ontogeny of galanin in the mouse embryo from embryonic day 10 (E10) to embryonic day 15 (E15). We hypothesize that galanin-like immunoreactivity (GAL-LI) is present during this stage of development, when organogenesis is occurring. The data, along with the high degree of conservation of galanin among species, imply that galanin is an important modulator of embryonic development. (Abstract shortened by UMI.)