- We study the effects of velocity-changing collisions (VCCs) on nearly degenerate forward four-wave mixing (FWM) in the presence of a strong pump. The calculations include the effects of both VCCs and phase-interrupting collisions. Explicit calculations and results for FWM in two-level systems for the case of strong collisions are presented in the Doppler limit. Effects of saturation on VCC-induced narrowing and those of VCCs on strong-field-induced resonances are discussed in detail. 1990 The American Physical Society.