Abstract An investigation is in progress using the beagle dog to investigate the short-term responses of the supporting tissues to different methods of root canal preparation. Atypical apical lesions have been observed in 4/27 teeth examined 7–14 days after instrumentation. These lesions were related to the apical foramina and were characterized by total cellular destruction. The centres of the lesions had densely staining material which appeared to be either damaged collagen or nuclear fragments. PMNLs were present at the periphery. The exact aetiology of these lesions is uncertain at present. The possibility of a previously unreported tissue reaction to sodium hypochlorite is being investigated further.