We present and discuss a theory for hydrodynamic effects in forced-Rayleigh-scattering experiments. The efficiency of phase-conjugate wave-front generation by forced Rayleigh scattering varies significantly with the nature of the solvent and the scattering angle. Acoustic effects become important for small scattering angles and times shorter than 0.5 ns under which conditions the simple thermal-diffusion theory fails. The phase-conjugate reflection efficiency predicted by our theory at short times is much smaller than that predicted by the thermal-diffusion theory, and thus need not obscure other quantum electronic processes. © 1983 The American Physical Society.