The relation between the duration of antidiuresis and sodium excretion and the route of administration of [8-lysine-vasopressin or [8-D-arginine]-deaminovasopressin to conscious rats was investigated. Both compounds were administered either intravenously through a chronically inserted catheter in the right jugular vein, or subcutaneously. The rats were then placed into individual metabolic cages and urine was collected quantitatively. Subcutaneous administration of both compounds resulted in a more prolonged antidiuretic response, lower sodium excretion and higher total urine osmolality. In order to achieve a comparable half-time antidiuresis, it was necessary to apply about 1000 times higher does of [8-lysine]vasopressin than those of [8D-arginine]deaminovasopressin. The relation between the duration of both antidiuresis and sodium excretion and the way of administration was, however, more pronounced in the case of [8-lysine]vasopressin. The excretion of potassium was increased after both compounds, the highest value being found after subcutaneous administration of [8-lysine]vasopressin.