Hepatocytes were isolated from human liver tissue by a two-step perfusion technique. They were treated with vasopressin, angiotensin, ATP and phenylephrine, which are known to be Ca(2+)-mediated glycogenolytic agents in rat liver tissue, and as a control, they were treated with the cyclic AMP-mediated hormones glucagon and isoproterenol. All agonists induce a time-dependent activation of glycogen phosphorylase. Glucagon and isoproterenol induce a somewhat higher degree of phosphorylase activation compared with vasopressin, angiotensin, ATP and phenylephrine, which all increase inositol tris-phosphate levels and have no effect on the cyclic AMP levels. The total activity of glycogen phosphorylase (a + b), amounting to 30 to 35 mU/mg protein, is found to be much lower than that found in rat liver tissue. Because only minor differences could be found, we conclude that the regulation of glycogen phosphorylase in human liver tissue is basically the same as that found in rat liver tissue.