Monoamine-containing neurons (dopamine and serotonin) have been reported to be localized among the cells in the inner nuclear layer (INL) of the chick retina. To examine the development of these neurons and whether they have an ability to synthesize serotonin (5-HT) or dopamine from exogenous precursors, we administered monoamines and their precursors in vitro as well as in situ and examined monoamine neuronal differentiation histochemically. In the chick retina, serotonin-containing neurons were found among amacrine cells. Two other types of serotonin-accumulating neurons were observed in the INL. One also had the ability to take up tryptophan and 5-hydroxytryptophan (5-HTP) and convert them to serotonin. In the monolayer culture from 61/2-day-old chick embryonic retinas, serotonin-containing neurons were observed on the 4th day of culture, increased in number until the 8th day, but had almost disappeared by the 15th day. After incubation of 8 day-cultures with 5-hydroxytryptophan, the number of serotonin-immunoreactive cells was at least twice that seen in untreated cultures. On the 20th day of culture, the same percentage of cells had the capability to take up serotonin, but the number of those which took up 5-hydroxytryptophan and decarboxylated it to serotonin had decreased markedly. Dopaminergic amacrine cells have also been found in the INL and shown to take up exogenously administered L-DOPA. In cultures, no dopamine-containing cells were observed with the present fluorescence histochemistry without prior incubation with DOPA.(ABSTRACT TRUNCATED AT 250 WORDS)