The participation of plasma catecholamine alterations in the development of renal hypertension is uncertain. Therefore, plasma catecholamines and phenylethanolamine N-methyl transferase (PNMT) activity in the adrenal gland were studied in rats with aortic ligation between the renal arteries. Blood pressure reached a plateau after 12 days (mean arterial pressure (MAP): 194 +/- 3 mmHg; P less than 0.001) and its elevation was accompanied by a biphasic elevation in plasma adrenaline. The first elevation (4-fold above control levels; P less than 0.001) occurred at 24 h after aortic ligation and lasted for 4 days. The second elevation commenced on day 6, reached its zenith at day 9 (16-fold increase; P less than 0.005) and lasted for 6 days. The first elevation was associated with the highest levels of plasma renin activity (PRA) (34-fold increase; P less than 0.001) and glucocorticoids (74% increase; P less than 0.001) but plasma noradrenaline, plasma dopamine and adrenal PNMT activity were minimally affected. However, a statistically significant increase in PNMT activity preceded and accompanied the second adrenaline elevation. Despite falling PRA and glucocorticoid levels, marked increases in plasma noradrenaline (5-fold increase; P less than 0.001) and plasma dopamine (2.5-fold increase; P less than 0.010) were observed. These experiments identify an early activation of the sympatho-adrenal axis in renal hypertension. Apparently there is a rapid release of the adrenaline pool followed by an elevation in PNMT activity. The results suggest that the sympatho-neuronal axis is also activated leading to increases in both plasma noradrenaline and dopamine levels.