The presence of endocrine cells in the gills of the goldfish Carassius carassius and the effects of lead intoxication (5 mg l(-1)) on their relative abundance and secretory activity were studied. Endocrine cells synthesizing peptido-like (met and leu-enkephalin and neuropeptide Y) and serotonin-like substances were detected in gill filaments by immunocytochemical procedures. Decreased immunoreactivity for two enkephalins and neuropeptide Y was observed after 48 and 96 h lead exposure. In contrast, increased serotonin immunoreactivity was evident after a one-week treatment. A search for Ca++-binding proteins was also carried out by immunocytochemical methods. Calbindin D-28K and S-100-like molecules were detected in gill epithelium, and their expression appeared enhanced after lead exposure.