We have observed experimentally the analog of the superfluid-helium fountain effect in a polycrystalline YBaCuO film. In the presence of a temperature gradient parallel to an electric current we have detected an asymmetry of the current-voltage characteristic. Reversing the direction of the temperature gradient also reverses the sign of the current asymmetry. The effect is most produced in zero magnetic field and is vanishing in a field of only a few Gauss. Our explanation is based on a two-fluid model by which the temperature gradient induces a counterflow of superconducting Electrons and quasi-particles. The resulting supercurrent generates a phase difference at the intra- and intergrain weak links and superimposes to the external current, thereby changing the critical value of the latter. From this thermally induced critical-current asymmetry we estimate the thermopower for the quasi-particles in the superconducting state of our sample.