1 The plasma availability of prednisolone after oral doses of prednisolone and its precursor, prednisone, were compared in ten normal controls and twenty-five patients with chronic active hepatitis by estimation of the area under the plasma concentration--time curve for the drug (AUC). 2 In controls, values for AUC were significantly more variable after prednisone than prednisolone, and two subjects showed markedly inefficient conversion of prednisone to prednisolone. In patients, variability was similarly wide after both preparations, but overall bioavailability after both prednisone and prednisolone was similar to that found in controls, although three patients showed subnormal values after both preparations, possibly as a result of impaired intestinal absorption. 3 Patients with biochemical and histological evidence of active hepatocellular necrosis showed evidence of impaired activation of prednisone, but this was compensated for by a decreased rate of elimination of prednisolone from the plasma. 4 It is concluded that plasma prednisolone levels will be more predictable after prednisolone than after prednisone in subjects without hepatic dysfunction. In the presence of liver disease, because of the marked variability in plasma prednisolone levels after either drug, estimation of these could be of value in those patients whose disease cannot be controlled by normal maintenance doses.