Cumulative copper poisoning was induced in rats by feeding a diet containing 2000 mg per kg copper as copper sulphate for 15 weeks. Animals were killed at intervals and their livers and kidneys examined histologically and analysed for their copper content. Subsequent events occurred in three phases, cumulative, crisis and recovery. The early period was associated with the gradual accumulation of copper in the liver and kidney and some signs of cellular disturbance; it culminated in a crisis characterized by maximal liver and kidney copper concentrations and severe cellular disruption. The final phase was one of regeneration and healing in which the animal demonstrated its tolerance to copper. It was concluded that long term copper dosing elicits a similar response in both the kidney and the liver of rats and that adaptation to copper takes place in both these organs, probably as a result of increased excretion and different modes of storage, which, in turn, contributes to the development of tolerance to copper.