Using immunocytochemistry, gastrin/CCK-like immunoreactivity is found in sensory nerve cells in the ectoderm of the mouth region of hydra and in nerve cells in the endoderm of all body regions of the sea anemone tealia. These results are corroborated by radioimmunoassay: One hydra contains at least 5 fmole and one tealia at least 2 nmole gastrin/CCK-like immunoreactivity. Reactivities towards gastrin and CCK antisera with different specificities suggest that the coelenterate gastrin/CCK-like peptide contains the C-terminal amino-acid sequence common to mammalian gastrin and CCK. In addition the radioimmunochemical data indicate that the coelenterate peptide also contains an amino-acid sequence that resembles the sequence 20-30 of porcine CCK-33, but that no other sequences of gastrin are present. Thus, it is probably more CCK-like than gastrin-like.