[en] The influence of the nuclear medium on the production of charged hadrons in semi-inclusive deep inelastic scattering has been studied by the HERMES experiment at DESY using 27.5 GeV positrons. A substantial reduction of the multiplicity of charged hadrons and identified charged pions from nuclei relative to that from deuterium has been measured as a function of the relevant kinematic variables. The preliminary results on krypton show a larger reduction of the multiplicity ratio R^h_M with respect to the one previously measured on nitrogen. Both the krypton and nitrogen data show that the multiplicity ratio is the same for positive and negative pions, while a significant difference is observed between R^h_M for positive and negative hadrons. This result can be interpreted in terms of a difference between the formation time of protons and pions. Alternatively, it has been suggested that the observed differences between positive and negative hadrons can be attributed to a different modification of the quark and antiquark fragmentation functions in nuclei