The WKB method is applied to wave propagation in stratified bi-isotropic media, or nonreciprocal chiral media, for waves with normal incidence. The method is based on the expansion of self-dual fields, which are certain combinations of the electric and magnetic fields. The propagating fields are derived with the WKB method. For sufficiently slowly varying media, the reflected fields are obtained as a first-order correction to the WKB solution. For normal incidence, the reflection effect is independent of the chirality parameter κ but depends on the nonreciprocity parameter Χ.