Since the isolation of galanin, a biologically active neuropeptide consisting of 29 amino acids pig intestine, the distribution, function, and biochemistry of galanin have been well studied in vertebrates. In contrast, studies of invertebrate galanin have been limited. We demonstrated the presence of galanin-like immunoreactivity in the central nervous systems of various molluscs including marine, freshwater, and land snails. In addition, galanin-like immunoreactivity was also detected in nerve nets of hydra and freshwater medusae which have the most primitive type of nervous system. This suggests that galanin is one of the phylogenetically oldest neuropeptides. Projections of the galanin-immunoreactive central nervous system to the salivary gland and buccal bulb were demonstrated in a snail, Indoplanorbis exustus, by retrograde transport experiments. It would be of interest to know the physiological functions of galanin in snail muscle. Western blot analyses of the central nervous system of I.exustus and the optic lobe of octopus indicated galanin-immunoreactive bands at 6.1 and 28.1 kDa, respectively. The positions of these bands were markedly different from that of vertebrate galanin (3.1 kDa), suggesting that the invertebrate and vertebrate galanins differ considerably. Interest will now be focused on the gene encoding invertebrate galanin.