[en] We present a new analysis of the ΔI=1/2 rule in K→ππ decays and the B_K parameter. We use the 1/N_c expansion in the effective chiral lagrangian for the pseudoscalar mesons and compute the hadronic matrix elements at leading and next-to-leading order in the chiral and the 1/N_c expansions. Numerically, our calculation reproduces the dominant ΔI=1/2 K→ππ amplitude. Our result depends only moderately on the choice of the cutoff scale in the chiral loops. The ΔI=3/2 amplitude emerges sufficiently suppressed but shows a significant dependence on the cutoff. The B_K parameter turns out to be smaller than the value previously obtained in the 1/N_c approach. It also shows a significant dependence on the choice of the cutoff scale. Our results indicate that corrections from higher order terms and/or higher resonances are large for the ΔI=3/2 K→ππ amplitude and the (Δvertical stroke S vertical stroke =2) K⁰- anti K⁰ transition amplitude. (orig.)