  The radiative decay of the massive neutrino $\nu_i \rightarrow \nu_j \gamma$
in a circularly polarized electromagnetic wave is investigated within the
Standard theory with lepton mixing. The decay probability in the wave field
does not contain a threshold factor $\sim ( 1 - m_i / m_j )$ as opposed to the
decay probability in a vacuum or in a constant uniform external field. The
phenomenon of the gigantic enhancement ( $\sim 10^{33}$ ) of the neutrino decay
probability in external wave field is discovered. The probability of the photon
splitting into the neutrino pair is obtained. (Published in Phys.Lett.B 321
(1994) 108).
