Idiosyncratic drug reactions are difficult to study in humans due to their unpredictability. Unfortunately, this characteristic also hinders the development of animal models needed for mechanistic studies. Nevirapine, used to treat human immunodeficiency virus (HIV) infections, results in a severe idiosyncratic skin rash in some patients. We found that nevirapine can also cause a significant rash in some strains of rats. At a dose of 150 mg/kg/day, the incidence in female Sprague-Dawley rats was 6/28 (21%), in female Brown Norway rats 32/32 (100%), and in female Lewis rats 0/6 (0%) while no male Sprague-Dawley or Brown Norway rats developed a rash. Female SJL mice 0/7 also did not develop nevirapine-induced skin lesions. The first sign of a reaction in Brown Norway rats was red ears at days 7-10 followed by a rash with scabbing mainly on the back; this was a shorter time to onset than in Sprague-Dawley rats. Light microscopy of the skin revealed a primarily mononuclear inflammatory infiltrate and lesions typical of self-trauma. Immunohistochemistry results suggest that the infiltrate was composed of CD4 and CD8 T cells as well as macrophages. A lower dose of either 40 or 75 mg/kg/day did not lead to a rash and, in fact, 2 weeks of the lower doses induced tolerance to the 150 mg/kg/day dose in female Brown Norway rats. A dose of 100 mg/kg/day resulted in rash in 2/4 (50%) of female Brown Norway rats. Rechallenge of Brown Norway rats that had been allowed to recuperate after a nevirapine-induced rash led to red ears in less than 24 h followed by hair loss and occasional skin lesions. Although the skin rash was less evident on rechallenge, microscopically, the cellular infiltrate was more prominent, especially surrounding the hair follicles. Moreover, there were lesions of interface dermatitis with apoptosis and satellitosis, indicative of a cell-mediated immune attack on the epidermis. While systemic signs of illness did not accompany the rash on primary exposure, on rechallenge, the animals appeared generally unwell and this forced sacrifice after 2 weeks or less of treatment. Importantly, splenocytes isolated from rechallenged animals were able to transfer susceptibility to nevirapine-induced skin rash to naïve female Brown Norway recipients, which was illustrated by a faster time to onset of rash in the recipients. The characteristics of this adverse reaction are similar to that seen in humans; that is, it is idiosyncratic in that it only occurs in some strains of animals, is delayed in onset, is more common in females, is dose-dependent, and appears to be immune-mediated. Therefore, it may represent a good animal model for the study of idiosyncratic drug reactions.