This review discusses phosphorylation of target proteins in the plasma membrane, the sarcoplasmic reticulum, and the myofilaments of cardiac and vascular smooth muscle cells known to be important in regulating contraction and relaxation. Substantial data have been obtained on the regulation of calcium uptake mechanisms of the sarcoplasmic reticulum in cardiac as well as vascular smooth muscle, suggesting a central role of phospholamban for relaxation in the cardiovascular system. As shown for cardiac muscle, the calcium release from the sarcoplasmic reticulum is also under β-adrenergic control via phosphorylation of the ryanodine receptor. The developmental switching from the skeletal to the cardiac isoform of the cardiac thin-filament protein troponin I marks the onset of β-adrenergic regulation of myofilaments through phosphorylation. Further examples for the genetic control of systems regulated by protein phosphorylation are presented. The review emphasizes the power of molecular-biologic approaches in combination with traditional methods to achieve significant progress in this field of cardiovascular research.