The time and dose-relationships of isoproterenol and lipolysis, force of contraction, cyclic AMP levels, protein kinase activity and phosphorylase activity were studied in perfused rat hearts. All five parameters were increased by isoproterenol over very similar concentration ranges. Dibutyryl cyclic AMP also increased phosphorylase activity, force of contraction and lipolysis. The study supports a role for cyclic AMP and protein kinase in myocardial lipolysis but suggests that a cyclic AMP-independent mechanism also exists.