To delineate plasma catecholamine responses to central volume expansion, four salt-replete healthy adults were subjected to 4 h of thermoneutral head-out water immersion (WI) and infusion of 2 liters normal saline (SI) over 4 h on two separate occasions. Each study was preceded and followed by a control hour. Both of these maneuvers resulted in significant increases in urinary sodium excretion and suppression of PRA and plasma aldosterone levels. During WI studies, plasma norepinephrine levels fell steadily from a prestudy value of 453 +/- 74 pg/ml to a nadir of 254 +/- 71 pg/ml (P less than 0.05) during the fourth immersion hour. In response to SI, plasma norepinephrine fell steadily from a prestudy level of 328 +/- 56 pg/ml to a nadir of 261 +/- 47 pg/ml during the fourth hour of infusion. Plasma dopamine levels rose and epinephrine levels were unchanged in response to WI as well as SI. When the mean urinary sodium excretion was plotted against the mean dopamine to norepinephrine ratio, there was a direct relationship in WI studies (r = 0.90) as well as SI studies (r = 0.92). These data suggest that plasma norepinephrine levels fall and plasma dopamine levels rise in response to volume expansion. These data also suggest that relative concentrations of dopamine vs those of norepinephrine may have a role in mediating natriuresis in response to volume expansion.