This study was carried out to evaluate the effects of infection and trauma on the organ of Corti and spiral ganglion cells following per-scalar cochlear electrode implantations in cats. The results showed that the incidence of infection with a per-scalar electrode inserted through an opening made directly into the apical and middle turns is low, provided the procedure is carried out aseptically with an antibiotic cover. Trauma could also be kept to a minimum by inserting the electrode into the scala tympani, taking care to insert the electrode without applying force, and ensuring that the first insertion is the only one. Furthermore, the tip of the electrode should be blunt, and its diameter smaller than the width of the scala tympani. It was noted that damage to the organ of Corti, and loss of spiral ganglion cells only occurred as a local phenomenon, and its extent was related to the severity of the damage to the Reissner's and basilar membranes. The insertion of an electrode through the round window and along the scala tympani was not free of risk, as infection which led to a marked loss of spiral ganglion cells occurred, particularly in the basal turn. On the other hand when the size of the electrode was small in relation to the cross sectional area of the scala tympani, and it was not inserted far along the length of the basal turn, the evidence of trauma was minimal. Tissue reaction to the teflon-coated platinum electrode was also not observed.