The rat adrenal medulla is immature at birth, composed of phaeochromoblasts and undifferentiated chromaffin cells, but by 7 d postnatally morphologically distinct adrenaline-storing (A) and noradrenaline-storing (NA) cells can be distinguished in the adult proportions of approximately 80-85% A and 15-20% NA cells. Glucocorticoid hormones are known to play an important role in the initial expression and maintenance of phenylethanolamine N-methyl transferase (PNMT), the enzyme characteristic of A cells. The purpose of the study was to investigate the effects of glucocorticoids on the establishment of the A and NA cell phenotype in vivo during the first postnatal week. Neonatal rats were treated from postnatal d 1 to 7 either with ACTH to increase circulating levels of glucocorticoids or with aminoglutethimide to reduce blood glucocorticoids. On postnatal d 7 the volume fractions of A and NA cells in the adrenal medulla were estimated and the amounts of stored adrenaline and noradrenaline determined by HPLC and compared with untreated controls. Adrenaline levels were increased following ACTH treatment and there was an apparent decrease after aminoglutethimide which was not statistically significant. There was cytological evidence of the effects of ACTH and aminoglutethimide on the adrenal cortex but no resultant effect on medullary cell morphology. A cells remained predominant with NA cells making up approximately 15% of chromaffin cells, suggesting that any effects of altered glucocorticoid levels were confined to a modulation of adrenaline synthesis by a morphologically unchanged chromaffin cell population.(ABSTRACT TRUNCATED AT 250 WORDS)