Diazepam (70-150 micrograms/ml) significantly inhibits oxygen consumption by pig kidney embryo cells and causes the cellular ATP level to fall. The maximum inhibitory effect develops after 1.5-2.5 h of diazepam treatment. In isolated mitochondria diazepam inhibits respiration in state 2 and 3u with glutamate and in state 3u with succinate. Ethylrhodamine staining and electron microscopic study reveal fragmentation of mitochondria in living cells.