An investigation has been made of flow phenomena associated with outflow from large horizontal cylindrical vessels with a vortex-free outlet on their bottom surface. This paper concentrates on the flow regimes and the variation of the level in the vessels with the flowrate. It is shown, both theoretically and experimentally, that there are two distinct regimes of outflow from the vessel, which are separated by a critical flowrate which is a strong function of the diameter of the outflow orifice. For the flowrates below the critical the level variation does not depend on the diameter of the outflow orifice, but the level variation depends strongly on it for the flowrates above the critical value. A model of transition between the flow regimes is also developed.