Abstract
In the framework of the Standard Model the probability (and time) of pi --> K transitions (oscillations) are computed. These transitions are virtual ones since masses of pi and K mesons differ considerably. These transitions (oscillations) can be registered through K decays after transitions of virtual K mesons to their own mass shell by using their quasielastic strong interactions. But for avoiding the background from inelastic K mesons, the energies E(pi) of pi mesons must be less than the threshold energy of their creation, i.e. E(pi) < 0.91GeV. The optimal distances for observation of these oscillations are computed. Solution of the problem of origin of mixing angle in the theory of vacuum oscillation is given.