Ad-enyl cyclase activity was determined in the 20,000 × g fraction of tran s-planted hepatoma and normal liver. The reaction mixture containing 0.5 ml of the enzyme, 4.0 ml of substrate (15.0 mg disodium ATP, 3.54 mg MgSO4, 2.08 mg NaF, 6.50 mg caffeine in 0 .05M Tris buffer, pH 7.2) was incubated 15 min at 37° and the reaction stopped by boiling for 3 min. In some experiments disodium ATP-3H was added to the reaction mixture and the cyclic AMP was measured by a radioisotope method. With the NaF present in the reaction mixture no stimulatory effect of epinephrine on adenyl cyclase could be detected in either normal or hepatoma tissue. It was assumed that NaF gave maximum stimulation of adenyl cyclase and so e pinephrine had no further stimulatory effect.