  A master equation approach to the study of environmental effects in the
adiabatic population transfer in three-state systems is presented. A systematic
comparison with the non-Hermitian Hamiltonian approach [N. V. Vitanov and S.
Stenholm, Phys. Rev. A {\bf 56}, 1463 (1997)] shows that in the weak coupling
limit the two treatments lead to essentially the same results. Instead, in the
strong damping limit the predictions are quite different: in particular the
counterintuitive sequences in the STIRAP scheme turn out to be much more
efficient than expected before. This point is explained in terms of quantum
Zeno dynamics.
