The breakdown of cortisone by the liver of Butazolidin treated rats as well as of untreated controls was examined. At the same time Butazolidin was determined in serum and liver at various times after its injection together with its distribution between incubation media and liver slices. The results are discussed in connection with former experiments in vitro in which Butazolidin in therapeutical doses was added to the incubation medium. From the distribution studies it appears that, in vivo as well as in vitro, the breakdown of cortisone is affected only at a certain Butazolidin concentration.