1. We have investigated the effects of ethanethiol, methanethiol and dimethyl sulphide on some metabolic processes of isolated rat hepatocytes, isolated mitochondria from liver and brain and ox-heart submitochondrial particles. 2. Ethanethiol, but not dimethyl sulphide, inhibited both gluconeogenesis and ureogenesis from various substrates in rat hepatocytes, depressed cellular ATP content and caused an increased reduction of the mitochondria. 3. Ethanethiol inhibited respiration in isolated rat-liver mitochondria with several substrates, both in the presence of ADP and phosphate or in the presence of an uncoupling agent. Ethanethiol also inhibited respiration in isolated rat-brain mitochondria. Dimethyl sulphide was much less effective in inhibiting mitochondrial respiration. 4. In ox-heart submitochondrial particles ethanethiol inhibited electron transfer between cytochrome c and oxygen. 5. Purified cytochrome c oxidase was inhibited by ethanethiol in a non-competitive manner. 6. Methanethiol inhibited cytochrome c oxidase and was an effective inhibitor of mitochondrial electron transfer, both in liver and brain. 7. The difference in inhibitory properties between ethanethiol, methanethiol and dimethyl sulphide observed in our experiments coincides with the difference in potency to elicit coma in rats. We suggest that inhibition of mitochondrial electron transfer by mercaptans may be relevant to the mechanism by which energy production in brain is depressed during hepatic coma.