This study provides details concerning the localization and role of 5-HT and DA in Schistosoma: 1) 3H-5HT is incorporated selectively by the nerve fibers containing dense granules measuring 820 +/- 20 A (type 2 granules), combined at times with other structures; the presence of "cold" dopamine in the environment does not alter the incorporation; 2) 3H-DA is also incorporated in vitro by the nerve fibers and occasionally by the muscle fibers of the adult Schistosoma; 3) neither the intraovular miracidium, the first and second generation sporocystes, nor the intrasporocystic cercariae appear to incorporate the 3H-5-HT or the 3H-DA, under our working conditions. The two amines are, however, incorporated by the hepatopancreas of Biomphalaria glabrata and the 3H-5-HT produced marking on certain neuromuscular zones of the tentacle.