The development of mitochondria was followed in black gram cotyledons cultured on filter paper wetted with water or benzyladenine solution. Treatment with benzyladenine brought about an increase in activities of some mitochondrial enzymes and in respiratory activities of mitochondria. Cycloheximide did not inhibit the increase, although it almost completely inhibited the incorporation of (14)C-leucine into mitochondrial protein. It was inferred that the increase in mitochondrial activity by benzyladenine-treatment was not due to biogenesis of mitochondria but due to an acceleration of activation processes in pre-existing mitochondria. The respiratory control ratio of mitochondria from benzyladenine-treated cotyledons was lower than that of mitochondria from untreated ones. It was also suggested that benzyladenine brought about some qualitative changes in mitochondrial activities.