Cultured human epidermoid carcinoma (HEp-2) cells were found to contain a highly responsive, catecholamine-sensitive adenyl cyclase activity in cellfree preparations. By contrast, cyclic AMP levels in intact HEp-2 cells were at best only marginally increased by catecholamines under a variety of conditions. The lack of an intact cell response could not be accounted for by escape of cyclic AMP to the medium, excessive phosphodiesterase activity, inactivation of the catecholamine, or by unusual kinetics of the system. However, in the presence of 1-methyl,3-iso-butylxanthine (MIX), a potent phosphodiesterase inhibitor, a moderate catecholamine response was observed in the intact cells. A significant elevation of cyclic AMP levels in the presence of MIX was observed at 0.3 muM epinephrine, and maximal levels occurred at 10 muM. Norepinephrine was much less effective than either epinephrine or isopropylnorepinephrine at 10 muM concentrations. In addition, intact cells slowly but steadily released cyclic AMP into the incubation medium over the course of 60-min incubations in the presence of MIX and epinephrine; maximum intracellular levels were reached by 5 min.