DURING an investigation of factors controlling rates of oxidation by mitochondria, it was found that variations of sucrose concentration in the suspending medium profoundly inhibited the oxygen consumption with some substrates but had much less effect with others. Earlier studies1,2 have indicated that the use of 0.88 M sucrose as the diluent during homogenization and isolation of mitochondria resulted in lower rates of mitochondrial oxidations than when 0.25 M sucrose was used. This was true despite the dilution of the mitochondria with other components of the final reaction mixture.