The diastereomeric forms of adenosine cyclic 3',5'-phosphorothioate, Rp cAMPS and Sp cAMPS, were studied in isolated hepatocytes from fed rats for their ability to interact with the intracellular cAMP-dependent protein kinase and to affect the phosphorylase kinase-phosphorylase glycogenolytic cascade. Incubation of the cells with increasing concentrations of Sp cAMPS produced a concentration-dependent activation of cAMP-dependent protein kinase with a concomitant increase in the glycogenolytic rate. Half-maximal and maximal velocities of glycogenolysis were reached at 8 X 10(-7) and 1 X 10(-5) M Sp cAMPS, respectively. Incubation of the cells with 10(-9) to 10(-4) M Rp cAMPS had no effect on basal glucose production or on cAMP-dependent protein kinase activity. Incubation of the cells simultaneously with 3 X 10(-6) M Sp cAMPS and increasing concentrations of Rp cAMPS produced half-maximal inhibition of glycogenolysis at 1 X 10(-5) M Rp cAMPS and maximal inhibition at 1 X 10(-4) M. The concentrations of Sp cAMPS required for half-maximal and maximal activation of glycogenolysis were increased 10-fold when 1 X 10(-5) M Rp cAMPS was present. These data imply that Sp cAMPS is a cAMP-agonist while Rp cAMPS is a cAMP-antagonist.