The interactions between nerve fibres from para- and orthosympathetic ganglia and adreno-cortical and -medullary cells were studied in joint cultures using explanted guinea-pig ciliary and sympathetic chain ganglia and enzyme-dispersed rat adrenal gland cells. Nerve fibres from both para- and orthosympathetic ganglia made only transitory contact with cortical cells, but consistently formed associations with isolated chromaffin cells which lasted for up to 10 days. Contacts between axons and chromaffin cells often showed particularly large varicosities and frequently withstood severe tests of durability from pulls of the fibre or the cell or both. By correlating phase contrast and catecholamine histochemistry (Falck-Hillarp method) it was shown that sympathetic fibres forming long-lasting contacts with chromaffin cells were adrenergic. The functional implications of the ability of autonomic nerves to distinguish between adreno-cortical and -medullary cells and the lack of specificity shown by the para- and orthosympathetic neurons during formation of long-lasting associations with chromaffin cells are discussed.