We present experimental results on the transport properties of a superconducting aluminum loop connected to reservoirs. As function of the applied magnetic field, an unexpected behavior is found-a steep enhancement (for temperatures close to the critical temperature) and a sharp drop (at lower temperatures) of the critical current at some value of the magnetic field B-a(*)(T). These effects are a consequence of a sudden suppression of superconductivity at B-a=B-a(*) in the banks to which the loop is attached by the current leads. As a result, the normal metal-superconductor boundaries appear at the ends of the current leads and the quasiparticle distribution function f(E) in the superconductor deviates strongly from its equilibrium value. Calculations based on the Usadel equations show that the critical current should be enhanced at high temperature (due to the penetration of the normal current from the normal metal-superconductor boundaries), whereas at low temperatures, the suppression of the order parameter by the nonequilibrium f(E) dominates and the critical current decreases. The latter provides a direct experimental verification of the recently proposed mechanism of the destruction of superconductivity by an applied voltage.