Following the initial discovery of adenosine 3',5'-monophosphate (cyclic AMP) dependent protein kinase (hereafter referred to as A-PK) in rabbit skeletal muscle (1), the cyclic AMP target enzyme was subsequently found to occur ubiquitously in mammalian (2) and nonmammalian (2-4) tissues. The molecular properties and biological roles of the enzyme have been extensively investigated; reviews dealing with them have appeared (5-7). Guanosine 3',5'-monophosphate (cyclic GMP) dependent protein kinase (hereafter referred to as G-PK), activated preferentially by cyclic GMP rather than by cyclic AMP, was later found to be present in every one of many arthropod tissues examined. Its concentrations are generally compara ble to those of A-PK also present in the same tissues (3, 4). One interesting tissue is the fat body of the silkmoth pupae and larvae, which was found to contain almost exclusively G-PK (4). The arthropod G-PK has many properties similar to those of A-PK of either mammalian or arthropod origins (3, 4, 8). Some differences, however, do exist between the two classes of protein kinases, such as specificity for substrate proteins (8-11) and differential effects of protein kinase modulators (12).