We studied the effects of P-adrenergic stimulation on the formation of adenosine 3',5'-cyclic monophosphate (CAMP) and on the cell volume in Rana ridibunda red blood cells under 0 2 (high oxygen atmosphere) and under N2 (hypoxia) gases. Adrenergic stimulation caused the accumulation of cAMP and the swelling of R. ridibunda erythrocytes. 10" M adrenaline and M isoproterenol seem to be the more efficient concentration in causing greater cell swelling. The amount of CAMP required for adrenergic swelling was minimal; swelling was observed at cAMP concentrations higher than 20 nM. Maximal swelling was seen when the cellular CAMP concentration reached 40 nM at high oxygen atmosphere. Isoproterenol caused a greater increase in water content than adrenaline. At hypoxic conditions the water content and CAMP concentrations obtained after stimulation with adrenaline and isoproterenol were greater than at high oxygen atmosphere. Since the adrenergic response seems to be blocked both by prazosin and propranolole the adrenoreceptors of R. ridibunda red cells may be classified as al-and P-receptors. The enhancement of CAMP and cell swelling may be either due to an increase in the number of adrenoreceptors, or to increased responsiveness of the erythrocyte Na'-H' antiporter to catecholamines during hypoxia. These responses are believed to enhance blood oxygen transport during periods of increased oxygen demands.