Doses of 1.66 mg Cu/kg/day, as cupric acetate in aqueous solution, were injected intramuscularly into the lateral thigh muscles of rabbits. The rate of loss of Cu from the injection site was estimated from in vivo measurements of 64Cu injected on the 15th day. Biological half-life values were 1.0 h for the first component (accounting for 65.2% of the 64 Cu) and 14.6 h for the second component (34.8% of the 64Cu). For the control group, values were 0.8 h and 62.2%, 14.6 h and 37.8%. Grossly visible lesions of dermatitis were noted on the paws, i.e. at sites removed from the site of injection, in some rabbits injected with Cu acetate two days post injectionem (p.i.). Histologically detectable lesions of acute inflammation were seen as early as 24 hours p.i. at the injection site of rabbits which had been exposed to the Cu once. Multiple injections and longer time periods resulted in lesions of acute and chronic inflammation. Cu was detected by the use of rubeanic acid stain. Signs of muscle degeneration and regeneration were seen as early as three days p.i. in rabbits which had received two injections of Cu. Rabbits which had been killed six days p.i. after a single injection showed chronic inflammatory changes and newly formed myofibres. Rabbits which had been killed 37 days p.i. after 17 injections showed lesions of acute and chronic inflammation of muscle and surrounding connective tissue, as well as signs of muscle regeneration. The gangrene visible grossly was attributed to the ischaemia caused by conglomerations of Cu.