The presence of neurokinin A immunoreactivity was studied in the chromaffin cells of the porcine adrenal medulla and in the nerve fibres innervating the adrenal gland during ontogenic development. For comparison, chromogranin A immunoreactivity was used as a marker for chromaffin cells. Whereas chromogranin A was found in chromaffin cells through all steps in embryonic development, three developmental stages of neurokinin A immunoreactivity could be distinguished. In the first and second trimester of gestation, neurokinin A was observed in some groups of chromaffin cells, but no neurokinin-immunoreactive nerve fibres could be detected. In the last trimester of gestation, neurokinin A-reactive chromaffin cells and nerve fibres were both found in adrenal glands. However, in adrenal glands of neonatal piglets, neurokinin A was found only in nerve fibres and not in chromaffin cells. From these results a hypothesis is proposed that neurokinin A might act as a neurotrophic factor in the early stages of the developing porcine chromaffin cells. Biochemical studies are being performed in order to confirm these morphological results and to study the possible role of neurokinin A as a neurotrophic factor in the adrenal gland.