Livers of mice on diets deficient in essential fatty acids (EFA) have been studied by light and electron microscopy. The most conspicuous changes occur in the mitochondria. In light microscopy the mitochondria appear very much enlarged in the periportal region of the lobule. In electron micrographs they have additional cristae, sometimes very abundant. ranged in stacks in the central cavity. The matrix may be more electron-opaque than normal. This is in contrast with the enlarged mitochondria appearing under other experimental conditions, where the cristae are reduced in number and the matrix is less electron-opaque. It is known that there is an uncoupling of oxidative phosphorylation in EFA-deficient mitochondria. As a hypothesis it is proposed that the uncoupling may be due to a molecular defect caused by the absence of EFA in the structure that determines the spatial relationship between the electron transport chain and oxidative phosphorylation. It is further tentatively suggested that the changes in mitochondria may be attributed to lack of ATP. The possibility is discussed that the mitochondrial changes are ineffective attempts at compensation for this lack.

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