Ultrastructural and spectrophotometric studies on mitochondrial swelling and contraction were carried out. All mitochondria just after isolated from rat liver showed condensed conformation. When mitochondria were incubated in 0.25 M sucrose only, they were moderately swollen and the absorbance at 520 nm was slightly decreased. Adding ATP to sucrose caused the absorbance to increase and the mitochondria to contract partially. KCl solution of 0.3 M induced marked decrease of absorbance and swelling of mitochondria. When ATP was added to KCl, increase of the absorbance and no contraction of the mitochondria excluding increased electron density of mitochondrial intermembranes were observed. Markedly decreased absorbance and somewhat largely swelled mitochondria in sodium arsenite solution of 0.4 or 1.0 mM were observed. When ATP was added to sodium arsenite, the absorbance increased slightly but mitochondria were more contracted than those in KCl-treated group. Above results indicate that the absorbance may not be correlated to morphological observations in the mitochondrial swelling and contraction.