  We calculate the different contributions to the decay $K_{L}\to \pi ^{0}\nu
\bar{\nu}$ that arise if the neutrinos are massive. In spite of a chiral
enhancement factor, we find that these contributions are negligibly small.
Compared to the CP violating leading contributions, the CP conserving
contributions related to Dirac masses are suppressed by a factor of order $(m_K
m_\nu/m_W^2)^2\lesssim10^{-12}$, and those related to Majorana masses are
suppressed by a factor of order $(\alpha_W m_K m_s^2
m_\nu/m_W^4)^2\lesssim10^{-29}$. With lepton flavor mixing we find new
contributions with a single CP violating coupling leading to a final CP even
state or with two CP violating couplings leading to a final CP odd state. These
contributions can be of the order of the Standard Model CP conserving
contributions to the flavor diagonal modes.
