We describe experiments on the spreading of liquid droplets deposited on a (wettable) liquid substrate, when the friction in the droplet dominates. We find that the horizontal radius R increases with the spreading time t like t(0.26+/-0.02) in the capillary regime (R < kappa(-1), the interfacial capillary length), and like t(0.51+/-0.02) in the gravity regime. These results are in good agreement with a recent theory.