Many years ago Richet (1902, 1905) demonstrated the presence of several toxic substances in extracts of tentacles of sea anemones, the study of which incidentally led to the discovery of anaphylaxis. Although several attempts have been made to identify these substances (Richet, 1905; Richet & Portier, 1936;Sonderhoff, 1936), none has been successful. Ackermann, Holtz & Reinwein (1923), however, identified tetramethylammonium in a sea anemone but did not determine its contribution to the toxic properties of anemone extracts.The present experiments were undertaken in an attempt to identify thalassine, the pruritogenic substance in coelenterates (Richet, 1902(Richet, , 1905 which is also a potent histamine liberator (Jaques & Schachter, 1954).We soon found, however, that a number of distinct pharmacologically active substances were present in sea anemones, and proceeded to identify them. We have, in fact, identified and determined the distribution of 5-hydroxytryptamine (5-HT), histamine, and tetramethylammonium (TMA); we have also found and studied the properties of another potent substance, probably a protein, which produces a marked and maintained contracture of the frog rectus abdominis muscle and a quick contraction, or twitch, of the sartorius muscle. Thalassine, however, is probably not identical with any of these.In addition to the above pharmacologically active substances we identified the apparently inactive base, homarine (N-methyl picolinic acid betaine), in high concentrations in the tissues of all the sea anemones which we studied, and also in the Portuguese man-of-war, Physalia.Homarine, which we found in very high concentrations in some tissues, derived its name from the fact that it was discovered in the tissues of the lobster, Homarus vulgaris (Hoppe-Seyler, 1933).