A comparative quantitative study on the occurrence of 5-HT in several tissues and organs of the elasmobranch Scyliorhinus stellaris and the teleost Conger conger has been carried out. 2. In Scyliorhinus, the richest source of the amine is the brain, in which 5-HT is twice as concentrated as in the teleost brain. Significant levels have been also detected in the intestine, followed by gills and heart. 5-HT is also concentrated in other epithelial organs, such as the rectal gland and the olfactory organ. 3. In Conger, the gills show the highest content of 5-HT, in which, like in the kidney, serotonin is 2-3 times more concentrated than in the corresponding organs of the elasmobranch. 4. These differences are discussed in relation to the distinct phylogenetic and ecophysiological features of the two animals. In addition, the possible functional significance of 5-HT in the fish heart is taken into consideration.
