The concentration of dopamine, and its metabolites 3,4-dihydroxyphenylacetic and homovanillic acids, as well as serotonin and its metabolite 5-hydroxyindoleacetic acid, were determined in the retina of two teleosts, C. aurufus (goldfish) and E. plumieri (mojarra), and two mammals, R. norvegicus (rat) and 0. cuniculus (rabbit). The turnover rate of these monoamines were investigated in the four species by the calculation of the ratio monoaminelmetabolite as an indirect index, and in goldfish and rat by the inhibition of the synthesis with a-methyl-p-tyrosine or p-chlorophenylalanine, by the increase in dopamine or serotonin by the corresponding precursors, 3,4dihydroxyphenylalanine or 5-hydroxytryptophan, and by inhibition of monoaminooxidase with pargyline. The modulation by light and dark stimulation was studied in the goldfish and the rat. Differences in the concentration and turnover rate were observed among the species. Serotonin concentration was higher in the teleosts. The administration of inhibitors of dopamine and serotonin synthesis differentially decreased the levels of the monoamines in the retina of goldfish and rat. The rate of formation of dopamine and serotonin by the corresponding precursors was much higher in the goldfish than in the rat. Pargyline administration decreased 3,4-dihydroxyphenylacetic and 5-hydroxyindoleacetic acids at different rates and time dependency in the retina of goldfish and rat. Dopamine and serotonin concentration did not exhibit high modifications by the inhibitor, suggesting the function of regulatory mechanisms or additional effect of pargyline at other sites different from monoaminooxidase. Light stimulation decreased the ratios of dopamine and its metabolites in the retina of goldfish, but not in the rat, probably by a greater release of dopamine with lower uptake capacity in the latter. Goldfish or rat retinal serotonin did not change by light exposure. Homovanillic acid was not detected in either type of retina after the dark period. In the goldfish, there was an increase in dopamine/3,4-dihydroxyphenylacetic acid ratio by the darkness; in the rat, this ratio decreased, which might be related to the release of dopamine. Retinal serotonin turnover rate was accelerated in the goldfish by dark stimulation. The present comparative results support a differential regulation of monoamine levels among species, and will be useful for further characterization of the regulatory mechanisms related to light and dark exposure.