- The possibility of formulating a treatment of pressure-induced extra resonances (PIER) in four-wave mixing, in terms of effective two-level equations, is examined. With the use of the method of adiabatic elimination of fast variables, effective two-level equations, which are valid for arbitrary slowly varying field envelopes, are obtained. Such equations are found to have a structure that is very different from that of the Bloch equations. In the new structure important inhomogeneous terms appear that are crucial for PIER. The present description is then used to study the effect of laser fluctuations on PIER. 1982 The American Physical Society.