The role of the kidney in handling the biologically acthe, nnconjugated endogenous catecholamines epinephrine (E), noreptnephrine (NE), dopamlne (DA), and the O-methylated metabolite of NE, normetanephrine (NM), was studied in the anesthetized dog before and after Toloroe expansion with iaotonk saline by measuring renal arterial deUrery, urinary excretion rate, and renal Tenons return of these materials. Net turnover in the renal metabolic compartment was estimated by comparing arterial delivery to urinary excretion and renal Tenons return. The kidney extracted E and produced NE, DA, and NM before and after Tolume expansion. After Tolume expansion a significant decrease in the extraction of E, an Increase in the production of DA, and no change in the production of NE or NM was seen. Clearance of catecbolamlnes and NM through the kidneys appeared to increase with Tolume expansion. The clearance of DA exceeded the clearance of creatinine (Cr) after volume expansion, while the clearance of NM exceeded that of Cr before and after volume expansion, indicating that urinary DA and NM are derived, in part, from processes other than glomerular filtration. These observations suggest an important role for the kidney in the modulation of the excretion of catecholamines and metabolites. This role must be considered before the urinary excretion rate of N, E, DA, and NM can be related to generalized sympathetic function. The observed increases in renal DA production after saline infusion suggest a possible natriuretic function of intrarenal DA. (Hypertension 2: 383-389, 1980)