For some parametric interactions with identically zero exponential gain for the signal wave the intensity of the idler wave can grow as a second power of the propagation coordinate. Such an amplification is revealed for the parametric mixing of four copropagating waves in BaTiO 3 ; two of them are ordinarily polarized and the two others are extraordinarily polarized. This mixing is used to build up a coherent oscillator. A reasonable qualitative agreement of the experimental results with the calculated data is demonstrated.