Our hypothesis that the strong effects of neutropic drugs on nervous system have hidden more widespread effects on all tissues and cells, is collecting evidences in our laboratory. We have essayed in vitro benzodiazepines on liver rat mitochondria. The present work deals with experiments on diazepam: it depresses the respiration coupled to phorphorylation in mitochondria having a good respiratory control; so respiratory control too is depressed. Also P/O ratio is lowered.