The regulation of glycogenolysis in human muscle during isometric contraction without and with adrenalin infusion has been investigated. The content of cAMP in muscle increased three‐fold during the infusion. Total glycogen phosphorylase and synthetase activities were unchanged during contraction without and with adrenalin infusion. The fraction of phosphorylase in theaform was in resting muscle 26% and at the end of contraction 24%. During adrenalin infusion phosphorylaseaincreased to 80%. Contraction during continued infusion resulted in a decrease of phsophorylaseato 42%, despite persistently increased cAMP content in muscle. The activity of synthetase I decreased to about half of the initial value during adrenalin infusion and contraction both without and with the infusion. The rate of glycogenolysis in muscle during contraction was not significantly changed by the infusion. Phosphocreatine (PCr) decreased during the contraction and the decrease was similar without and with adrenalin infusion. The amount of inorganic phosphate (P1) accumulated in muscle during contraction was lower when adrenalin was given due to a greater accumulation of hexosemonophosphates. It is concluded that the rate of glycogenolysis in muscle during contraction without and with adrenalin infusion is a function both of phosphorylase in the formaform and the availability of Pj at the active site of the enzym