We have previously shown that the neural crest is regionalized early into "adrenergic" and "cholinergic" areas from which arise, respectively, the sympathetic and parasympathetic ganglioblasts of the autonomic nervous system. This regionalization does not correspond, however, to an irreversible determination of the neural crest cells since, under certain experimental conditions, cholinergic cells can arise from the adrenergic region of the crest and vice versa. The phenotypic expression of the presumptive ganglion cells appears to be responsive to the environmental conditions they encounter during and/or after their migration. In the present study we show that the developmental behavior of parasympathetic ganglion cells which have stopped migrating and at least some of which have started to differentiate into cholinergic neurons can be profoundly modified if they are transplanted into a younger embryo at the trunk neural crest level. The crest level. The grafted ganglion cells start migrating and stop in the same sites as the host neural crest cells. Their further differentiation depends on their localization. When situated in the adrenergic ganglia and in the suprarenal gland they synthesize catehcolamines, whereas they differentiate into nonfluorescent, silver-staining ganglion cells if they migrate in the gut wall. Thus, the differentiation of autonomic neurons is dependent on tissue interactions even after the neural crest cells have grouped to form ganglionic structures in which biochemical differentiation is already in progress.