We have previously shown that rat enteric neurons display many of their in vivo phenotypes when they are dissociated and grown in long-term cell culture. To assess the degree of plasticity of these phenotypes we have examined the effect of medium conditioned by rat heart cells because this treatment strongly affects transmitter properties in rat sympathetic neurons in culture. Growth of enteric neurons for 3-4 weeks in conditioned medium caused several changes that are similar to previously described effects of conditioned medium on other neuronal cell types in culture. When compared to cultures grown in control medium, cultures grown in conditioned medium: (i) contained three times as many large (greater than 25 micron) neurons; (ii) synthesized and stored 3-4 times as much acetylcholine; (iii) contained 4-5 times as many neurons with detectable 5-hydroxytryptamine immunoreactivity; and (iv) contained 10 times as many neurons that fired repetitively during sustained depolarization. Several other changes, which have not been reported in other systems, were also observed. Conditioned medium cultures: (i) contained many fewer neuronal processes with immunohistochemically detectable vasoactive intestinal polypeptide, substance P, somatostatin, and [Met]enkephalin; (ii) contained 70% fewer neuronal cell bodies with vasoactive intestinal polypeptide-like immunoreactivity; and (iii) contained four times as many neurons that had muscarinic responses to acetylcholine. None of the changes in properties described above uniformly affected all enteric neurons, even after 6 weeks of growth in conditioned medium. We conclude that the heterogeneity of enteric neuron phenotypes is established prior to birth and limits the capacity of certain subsets of neurons to respond to exogenous factors in the environment. Nevertheless, the phenotypes of other subsets of neurons displayed considerable plasticity when exposed to conditioned medium.