SUMMARY The acute effects of vasopressin (40 mU) were studied in the conscious sheep. Vasopressin caused consistent increases in K and C1 excretion and variable increases in Na and total solute output. These changes occurred within 10 min., reached a peak within 30 min. and subsided within 60 min. When the urine was initially dilute (< 800 μosmoles/ml.) vasopressin caused an antidiuresis. When the original concentration exceeded 1000 (μosmoles/ml. vasopressin caused small increases in urine flow. Vasopressin always increased water reabsorption relative to solute reabsorption.