Protein phosphorylation is now recognized to be the major general mechanism by which intracellular events in mammalian tissues are controlled by external physiological stimuli. However, only recently has the idea that different cellular functions are controlled by common protein kinases and protein phosphatases started to gain widespread acceptance. Thus there is an integrated network of regulatory pathways, mediated by phosphorylation-dephosphorylation, that allows diverse cellular events to be coordinated by neural and hormonal stimuli. The evidence that supports this concept is reviewed, with emphasis on the role of protein phosphorylation in enzyme regulation.