  The observed neutrino oscillations lead to the conclusion that the neutrinos
have a non zero mass, whereas the standard calculations of the weak leptonic
decays assume they are massless. However, the experimental uncertainties in the
quantities observed in such decays are able to absorb the effects of the non
zero neutrino masses, provided that they are not too large, as it seems to be
according to the presently established upper limits. This fact is shown in
detail for the $\mu$ decay, but it can be established also for the leptonic
decays of the tau meson.
