The presence of noradrenaline (NA) and the possible interaction of serotonin (5-HT) with dopaminergic and noradrenergic neuronal elements was studied in the retina of the teleost Eugerres plumieri. By means of a histofluorescent technique, paired amacrine cells can be visualized after intravitreal injection of NA or 5,6-dihydroxytriptamine, suggesting their probable catecholaminergic and indoleaminergic nature. With a high-performance liquid chromatographic (HPLC) method and after pargyline treatment of the animal, 6.86 ng/mg protein of dopamine (DA) was detected, while NA content was 0.50 ng/mg protein. The NA levels of the retina increased, whereas the DA levels decreased in a significant manner after in vivo treatment with 5-HT. 5-methoxy-N,N-dimethyltryptamine, a 5-HT agonist, was able to mimic this effect partially, while the agonists tryptamine and quipazine did not affect the levels of DA and NA. The antagonists methysergide, metergoline, and cyproheptadine significantly blocked the 5-HT-induced NA increase, whereas only the first two antagonists were effective in inhibiting the 5-HT-induced DA decrease. The 5-HT modulation of NA and DA is apparently receptor mediated and is not due to a hetero-exchange, since imipramine was not able to block the 5-HT effect. These findings support the suggested interaction between serotoninergic and dopaminergic cells [Negishi et al: J Neurosci Res 5:621-635, 1980; Neurosci Lett 25:1-5, 1981]. Furthermore, they demonstrate the possible modulation which 5-HT exerts on the endogenous levels of NA.