~o ~a ,~ so 6o 2~ ~o ~o 5o 6o 2o ao ,~ so 6o ~o 3o ~o ~o ~ port to the synchronous variations under- gone by the two cyclic nucleotides in bio- logical systems (7). Moreover, the varia- tions in protein kinase activity during the same experiment lend further support to the idea stated above: the similar trends in the variation of the kinase and cAMP levels agree with the established concept that cAMP stimulates the production of a cAMP-dependent protein kinase, thereby inducing protein phosphorylation. In con- clusion, these results support the idea that the cAMP-protein kinase system may be involved in the heterophylic action of ec- dysterone.