Composed mainly of hormone-producing chromaffin cells, the adrenal medulla is the principal site of the conversion of the amino acid tyrosine into the catecholamines adrenaline (epinephrine) and noradrenaline (norepinephrine). Medullary cells are derived from the embryonic neural crest and, as such, are simply modified neurons. In particular, they are modified postganglionic cells of the sympathetic nervous system that have lost their axons and dendrites, receiving innervation from corresponding preganglionic fibers. Moreover, as the synapses between pre- and postganglionic fibers are called ganglia, the adrenal medulla is actually a ganglion of the sympathetic nervous system.

In response to stressors such as exercise or imminent danger, medullary cells release catecholamines into the blood in a 70:30 ratio of adrenaline to noradrenaline. Notable effects of adrenaline and noradrenaline include increased heart rate, blood vessel constriction, bronchiole dilation, and increased metabolism, all of which are characteristic of the fight-or-flight response.