Using morphometric analysis and high-pressure liquid chromatography with electrochemical detection of catecholamines the volumes of adrenaline-storing (A) and noradrenaline-storing (NA) cells and the adrenaline and noradrenaline contents of adrenal glands of normal and age-matched hypophysectomized rats were determined and compared. Some hypophysectomized rats were also administered 150 mg metyrapone/kg daily for a 4-week period. Hypophysectomy resulted in an increase in noradrenaline storage in adrenal glands. This was more marked in hypophysectomized animals not treated with metyrapone even though the combination of metyrapone and hypophysectomy resulted in a reduction of blood corticosterone concentration to only approximately 6% normal. There was no evidence of a change in proportion of A and NA cells in the adrenal medulla of hypophysectomized rats or hypophysectomized animals treated with metyrapone and it is concluded that the changes in amine concentrations and proportions observed reflect an increase in the storage of noradrenaline in A cells as a consequence of less efficient methylation of noradrenaline after hypophysectomy. The findings also showed that the left adrenal gland and medulla was usually slightly larger and had a greater catecholamine content and slightly higher proportion of NA cells than the right adrenal gland. During the period of the experiment (animals aged 6-15 weeks) there was an increase in the concentration of both adrenaline and noradrenaline in the normal adrenal glands. In normal Sprague-Dawley rats weighing 140 and 400 g the adrenal medulla accounted for some 5% of adrenal volume; this increased to 13% after hypophysectomy; A and NA cells together accounted for 50-60% of the volume of adrenal medulla.