The influence of muscle contraction, induced by electrical stimulation, on the activity of glycogen phosphorylase, the contents of high-energy phosphates, hexose-monophosphates and lactate have been studied in isolated extensor digitorum longus (EDL) and soleus muscles from rats. The activity of phosphorylase a + b was about nine times higher in fast twitch muscles (EDL) than in slow-twitch soleus and remained unchanged during the stimulation. A pronounced increase of phosphorylase a occurred during the stimulation in EDL muscle. Stimulation with a frequency of 50 Hz for 10 s and 2 Hz for 90 s resulted in a 44-fold and five-fold increase in phosphorylase a, respectively. In contrast, stimulation of soleus muscle resulted in only a minor increase of phosphorylase a. The rate of glycogenolysis increased in both muscles during the stimulation but the increase was four to five times higher in the EDL than in soleus muscle. The content of phosphocreatine (PCr) before stimulation was much higher in EDL than in soleus but similar after the stimulation. This resulted in a three- to four-fold higher release of inorganic phosphate (Pi) in EDL than in soleus during contraction. Pi has previously been shown to be present in a limiting amount for the activity of phosphorylase and the increase during contraction is of importance for increasing the glycogenolytic rate. It is concluded that the higher glycogenolytic capacity in fast-twitch muscles compared to slow-twitch muscles is due to: (1) higher content of phosphorylase a + b, (2) higher degree of transformation of the enzyme into the a form during contraction, and (3) higher content of PCr, which liberates a large amount of Pi during contraction.