The renal hemodynamic and excretory effects of clonidine were tested in 2 groups of dogs. In one group, the drug was given directly into the renal artery at a rate of 1.2 mug/min and resulted in a significant decrease of the effective renal plasma flow (ERPF) in both kidneys, an increase in filtration fraction (F.F.), urine volume (U.V.), and free water clearance (CH2O) and had no effect upon the glomerular filtration rate (GFR), osmolar clearance (Cosm) and the excretion of sodium (UNa+V), chloride (UC1-V), potassium (UK+V), calcium (UCa++V) and phosphorous (UP04V). In the second group of animals it was given intravenously at a rate of 12.0 mug/min and resulted in a significant decrease of ERPF, UNa+V, UC1-V, and an increase in F.F., U.V. and CH2O and had no effect upon GFR, Cosm, UK+V, UCa++V and UPO4V. Systemically, it decreased heart rate (H.R.) and respiratory rate (R.R.) in both groups of animals; it increased blood pressure (BP) in Group I and had no effect on BP in Group II.