  Flow equations describe the evolution of the effective action $\Gamma_k$ in
the process of varying an infrared cutoff $k$. The presence of the infrared
cutoff explicitly breaks gauge and hence BRS invariance. We derive modified
Slavnov-Taylor identities, which are valid for nonvanishing $k$. They guarantee
the BRS invariance of $\Gamma_k$ for $k\to0$, and hence allow the study of
non-abelian gauge theories by integrating the flow equations. Within a
perturbative expansion of $\Gamma_k$, we derive an equation for a $k$ dependent
mass term for the gauge fields implied by the modified Slavnov-Taylor
identities.
