  Radiative corrections in the order ${\alpha\over{2\pi}}{{m^2_e}\over
m^2_\mu}$ to $\mu $- and ${\alpha\over{2\pi}}{{m^2_\mu}\over m^2_\tau}$ to
$\tau $- decays are calculated. The decay width is enhanced by $4.48\cdot
10^{-3} ({\alpha\over{2\pi}})$ in the muon case and by $0.283
({\alpha\over{2\pi}})$ for the $\tau \rightarrow \mu \nu_\tau \bar\nu_\mu
(\gamma)$ decay. Influence of these corrections on the electroweak data is
discussed.
