A population of proliferating cells in the fetal gut has previously been found to express transiently aspects of a catecholaminergic phenotype (TC cells) during development in both rats and mice. These cells appear to be noradrenergic in rats but dopaminergic in mice. In the current study, the ability of TC cells, identified by the immunocytochemical demonstration of tyrosine hydroxylase (TH), to take up and become radioautographically labeled by [3H]norepinephrine ([3H]NE) was assessed. When TC cells were most numerous in the bowel of rats, no cells were labeled by [3H]NE (days E12 and E13). In rats, but not mice, labeling of larger cell bodies by [3H]NE was found on days E14 and E15. However, no cells showed TH immunoreactivity on day E15, although a few cells were doubly labeled by [3H]NE and TH immunoreactivity on day E14. Therefore, in rats TC cells contain TH immunoreactivity but do not take up [3H]NE prior to day E14, and their disappearance is followed by the appearance of a second population of larger cells that lacks TH immunoreactivity but which does take up [3H]NE. The transient appearance of some cells that express both markers on day E14 suggests, but does not prove, that TC cells change their phenotype and are the precursors of the cells found later in development that lack TH but which take up [3H]NE. The cells that take up [3H]NE are rare or absent in newborn rat gut, indicating that they may also be transient. These results indicate that genes responsible for different aspects of the noradrenergic phenotype need not necessarily be coupled in their expression. Although uptake of [3H] NE into cell bodies was not found on day E13 or later in vivo in mouse gut, it does occur in mouse bowel explanted prior to day E13 and grown for 10 to 12 days in culture. These cultures also contained TH immunoreactive cells. Thus, the potential for development of cells able to take up [3H]NE exists in mice as well as in rats, and the conditions that lead to a loss of catecholaminergic traits in vivo do not exist in vitro.