The writer reports experiments done with distilled water and hypotonic and hypertonic salt solutions of definite osmotic concentrations. The experiments were performed according to the Laewen-Trendelenburg technic using the vascular system of the frog's hind legs, and according to the Pissemski-Krawkow method using the capillaries of the rabbit's ear. Both preparations react to distilled water by marked vaso-constriction, the same phenomenon taking place in the case of the hypotonic salt solutions. The lower the concentration pf the hypotonic salt solution the stronger the vaso-constriction obtained. With hypertonic salt solutions was observed a strong but rather transient vaso-dilatation followed by secondary vaso-constriction. The later results were found only in the experiments with the frog's hind legs.