The distribution of neuropeptides immunologically related to vasoactive intestinal peptide (VIP) and its precursor peptide preproVIP(111-122), as well as to other peptides of the VIP-family, was studied in the central and peripheral nervous and sensory system of the snail, Helix pomatia, by use of immunocytochemical methods. VIP and preproVIP immunoreactivity was present in somata and nerve fibres of all central ganglia. Hibernating snails contained on average a total of 670 VIP- and 763 preproVIP-immunoreactive neurons. The number of immunoreactive cells was substantially reduced by more than 50% in active snails during summer with an average of 289 VIP- and 356 preproVIP-immunoreactive neurons. Antiserum against VIP labelled nerve fibres next to blood vessels and smooth muscle cells, whereas preproVIP-like material was localized in nerve fibres and endocrine-like cells among dorsal body cells and in the connective tissue along fiber tracts. VIP-immunoreactive material was also found in accessory ganglia of small and large tentacles, ganglia of the lips, the sensory epithelium of the tentacles, free nerve endings between skin epithelial cells, neuronal cells in the retina and in the sensory epithelium of statocysts. The cell-specific distribution and the seasonal variation of VIP- and preproVIP-like peptides suggest that they may act as transmitters or modulators in the nervous and sensory system and may be involved in the physiological adaptation of central neurons during long-term resting periods of snails.