The allosteric sensitivity of heart specific and skeletal muscle specific phosphorylase a-s from rabbit, pig, bovine and dog was compared. 2. 0.2-0.3 mM glucose 6-phosphate exerts 50% inhibition on the heart specific phosphorylase a-s without any inhibitory effect on the skeletal muscle specific ones. 3. AMP decreases the Km for substrates (glucose 1-phosphate, or Pi) of all heart specific phosphorylase a-s. It also enhances the Vmax of pig and bovine phosphorylase a-s 2-fold, but has no effect on the Vmax of rabbit and dog enzymes.