A variant of zero-angle light scattering is proposed with particular emphasis on the use of this technique for measuring chemical reaction rates. The system analyzed consists of a laser oscillator with a reacting sample placed inside the resonant cavity. Fluctuations about equilibrium of the chemical reaction frequency modulate the oscillator, and the power spectrum of the modulating signal is Lorentzian with width proportional to the reaction rate. The modulation signal is very weak, but should be detectable with modern techniques.