Super-Kamiokande has reported the results for the lepton events in the atmospheric neutrino experiment. These results have been presented for a 22.5kT water fiducial mass on an exposure of 1489 days, and the events are divided into sub-GeV, multi-GeV and PC events. We present a study of nuclear medium effects in the sub-GeV energy region of atmospheric neutrino events for the quasielastic scattering, incoherent and coherent pion production processes, as they give the most dominant contribution to the lepton events in this energy region. We have used the atmospheric neutrino flux given by Honda et al. These calculations have been done in the local density approximation. We take into account the effect of Pauli blocking, Fermi motion, Coulomb effect, renormalization of weak transition strengths in the nuclear medium in the case of the quasielastic reactions. The inelastic reactions leading to production of leptons along with pions is calculated in a $\Delta $- dominance model by taking into account the renormalization of $\Delta$ properties in the nuclear medium and the final state interaction effects of the outgoing pions with the residual nucleus. We present the results for the lepton events obtained in our model with and without nuclear medium effects, and compare them with the Monte Carlo predictions used in the simulation and the experimentally observed events reported by the Super-Kamiokande collaboration.

MiniBooNE and K2K collaborations have recently reported the ratio for the $\nu_\mu$ induced charge current 1$\pi^+$ production cross section to the charged current induced quasielastic lepton production cross section i.e. $R(E)=\frac{\sigma^{CC1\pi^+}}{\sigma^{CCQE}}$. In this paper we study, theoretically, in a local density approximation the effect of nuclear medium in charged current quasielastic(CCQE) and charged current one pion production(CC1$\pi^+$) processes for the $\nu_\mu$ induced reaction in nuclei. The theoretical results have been compared with the results of experimental observations by the MiniBooNE and K2K collaborations. The ratio R(E) for $\nu_\mu$ induced reaction in water for the proposed T2K experiment is also presented.