The putative central catecholamine neurotransmitters, adrenaline, noradrenaline and dopamine, have been quantified by enzyme radiochemical methods in the hypothalamo-neurohypophysial system (HNS) of the xeric rodents, Notomys alexis and Pseudomys australis, using the laboratory rat for comparison. All three catecholamines were found (noradrenaline greater than dopamine greater than adrenaline) in the hypothalami of the three species. The hypothalami of the two xeric rodents contained some 5-10 times as much of each of the catecholamines (when expressed as ng per body weight) as the rat. The ratio of noradrenaline to dopamine was constant (3 X 8-4 X 2) between species. Posterior pituitary contents of the catecholamines were generally inconsistent within and between species, with only dopamine being invariably and consistently present. It was also found that, like the antidiuretic hormone content, the dopamine content of the neural lobe was significantly correlated with the body surface area of the animal. Withdrawal of drinking water or its replacement with 2 X 5% saline increased hypothalamic noradrenaline synthesis and dopamine utilisation. In the neural lobe, saline ingestion produced a significant increase in all three catecholamines, but with water deprivation only dopamine was increased (relative to body weight). The results of these studies are interpreted as indicating a possible role for catecholamine neurotransmitters, particularly dopamine, in the control of neurohypophysial hormone release in the HNS.