Ultrastructural alterations of the liver were examined on the 1st, 2nd, 3rd, 5th, 7th and 14th day after a standard burn in mice. Our results can be divided pathogenetically into two groups: 1. Alterations explained by the primary thermal injury including electron-optically empty vacuoles and sinusendothelium destruction. 2. Alteration due to the influence of a specific burn toxin including mitochondrial changes and the formation of autophagic vacuoles. Evidence of this distinction was produced by a second investigation. Burn toxin isolated from the serum of severely burned patients was injected intraperitoneally (15 mg) into 4 mice. The livers were examined by electron microscopy on the 2nd, 4th, 7th and 14th day. Alterations described in 2 above were not observed in any case of this investigation series. The nature and timing of liver alterations were similar to those described in 2 above. In none of the cases were changes similar to those in 1 above observed.