DISPERSIVE ANALYSIS OF THE DECAY ETA-]3-PI

We demonstrate that the decay eta --> 3 pi represents a sensitive prob e for the breaking of chiral symmetry by the quark masses. The transit ion amplitude is proportional to the mass ratio (m(d)(2) - m(u)(2))/(m (s)(2) - (m) over cap(2)$). The factor of proportionality is calculate d by means of dispersion relations, using chiral perturbation theory t o determine the subtraction constants. The theoretical uncertainties i n the result are shown to be remarkably small, so that eta-decay may b e used to accurately measure this ratio of quark masses.