Mouse adrenal medulla was transplanted to mouse brain for morphological and morphometric examination of the nerve endings abutting on the surface of the grafted adrenal chromaffin cells. To determine the types of these endings, they were treated with antibodies specific for phenylethanolamine N-methyltransferase (PNMT), choline acetyltransferase (ChAT) and acetylcholinesterase (AChE). Three types of vesicles were found in nerve fibers and endings: the first contained small clear synaptic vesicles 30-50 nm in diameter, the second was mixed with large granules with moderately electron-dense cores 80-100 nm in diameter, and the third exhibited small electron-dense cored vesicles 50 nm in diameter. The two first types occurred in nerve endings of normal and grafted medulla, but the third was only seen in the grafts. Grafted chromaffin cells carried two morphologically distinct types of synapse: small with a diameter of 1-2 microns, and large, as in normal adrenal medulla. The first type predominated after transplantation. In normal medulla, the number of synapses calculated per grafted chromaffin cells was about 4.5 for cells containing epinephrine (E) and 5.8 for those containing norepinephrine (NE), and in grafted medulla, 4 per cells. After grafting, nerve endings were labeled to ChAT, AChE and neuron-specific enolase (NSE), but only a few nerve fibers were immunoreactive to PNMT. The presence of NSE in nerve endings on the grafted cells, a marker of the glycolytic activity in neurons, suggests the formation of de novo functional synaptic connections.