Treatment of P19 embryonal carcinoma cells with retinoic acid induces their differentiation into a population of cells consisting of neurons and other cell types normally derived from neuroectoderm. We used immunohistological and histochemical techniques to identify some of the neurotransmitters in the P19-derived neurons. The majority of neurons contained GABA, glutamic acid decarboxylase, and GABA-transaminase. Neuropeptide Y and somatostatin were less frequently found and both were partially co-expressed with GABA and with one another. Smaller numbers of cells were positive for tyrosine hydroxylase, DOPA decarboxylase, serotonin, calcitonin gene-related peptide, galanin and substance P. The variety and proportions of cells with different transmitter types were reproducible from one experiment to the next and varied very little over 40 days in culture except for cells containing enkephalin, which were abundant only in mature cultures of 32 days or more. Synapses formed between neurons and some contained both small clear and large dense-core vesicles within the presynaptic bouton. Because GABA, neuropeptide Y and somatostatin are abundant in P19-derived neurons as well as in embryonic neurons in rostral regions of the mammalian CNS, we suggest that the developmental events occurring in P19 cell cultures closely resemble those of the embryonic neuroectoderm.