Results are presented of calculations on the experimentally observed lifetime enhancement of high principal quantum number nf(N+ = 2) and np(N+ = 0) Rydberg states of the NO molecule in small dc electric fields. A Hund’s case (d) molecular Hamiltonian is introduced, which describes intramolecular interactions between low angular momentum Rydberg states as a result of core penetration and l mixing caused by the electric field. Eigenstates of the molecule in the electric field are obtained by diagonalization of the Hamiltonian and an effective decay time is determined which describes the multiexponential decay of the superposition of (Stark) states which is excited experimentally. The calculations reproduce the striking onset of the lifetime enhancement of the np(N+ = 0) series, which was observed experimentally. Both for the nf(N+ = 2) and np(N+ = 0) Rydberg series, core penetration by low-l states leads to extensive interseries coupling for several values of the principal quantum number.