Cyclic adenosine 3',5'-monophosphate (cAMP) accumulation and cation transport were measured in duck erythrocytes after stimulation by norepinephrine (NE) or shrinkage induced by exposure to hypertonic media (S). Previously both NE and S were shown to initiate a similar transport process in this cell. NE elicited a rapid rise in cellular cAMP and 42K influx. Both effects were eliminated by propranolol. At concentrations of NE below 3 X 10(-8) M (the concentration at which 42K influx saturates), there was good correlation between the magnitude of the permeability change and the increment in cAMP. In contrast, medium hypertonicity, at a level which stimulated K influx to the same extent as a near-maximal norepinephrine response, did not alter cAMP content. The data are discussed in terms of a model in which S and NE activate a final common transport pathway by different mechanisms, which in the case of S does not involve cAMP.