Chronic morphine administration in adult rats results in neurogenic secretion of adrenal catecholamines and compensatory increases in basal catecholamine levels, in activities of catecholamine biosynthetic enzymes and in the number of storage vesicles in the tissue. Perinatally addicted developing rats demonstrated changes completely different from those seen in adults; catecholamine levels and dopamine beta-hydroxylase activity were reduced compared to controlscsnnofinduction of tyrosine hydroxylase was observed. The time course of adrenomedullary maturation was delayed throuth the first 10-20 days of age, with reduced numbers of storage vesicles and larger proportions of partially filled vesicles. On exposure to morphine, continued until weaning, perinatally addicted rats did not display any of the changes in catecholamine synthesis or uptake seen in adult rats. The differences between adults and developing rats can be partly explained by the absence of functional innervation of the neonatal adrenal medulla.