The B-CK isozyme of cytoplasmic creatine kinase is localized distinctly in the terminal web region of the intestinal epithelial cell brush border (Keller and Gordon: Cell Motil. Cytoskeleton 19:169-179, 1991). Experiments were performed to determine whether this CK is energetically coupled to the myosin II that is present in the circumferential ring and interrootlet structural domains of the brush border terminal web. In isolated brush borders, ATP-dependent circumferential ring contraction and interrootlet myosin solubilization were supported either by an exogenous PEP-pyruvate kinase-based ATP-regeneration system (PEP-PK) or by the addition of phosphocreatine to the endogenous B-CK-based ATP-regeneration system (PCr-B-CK). Addition of an exogenous hexokinase-glucose ATP-hydrolysis system (HK-G) effectively blocked both contraction and myosin solubilization in the PEP-PK assay. In contrast, HK-G had no significant effect on PCr-B-CK-supported brush border contraction, although it did inhibit interrootlet myosin solubilization. Thus, when high-energy phosphate is supplied as phosphocreatine, brush border B-CK imparts to the circumferential ring myosin a selective energetic advantage over other ATPases. These results suggest that myosin and B-CK are functionally coupled in the brush border circumferential ring, where they might comprise one end of an energy circuit that supplies energy for contraction, but that colocalization of CK with myosin in the brush border interrootlet domain is insufficient to establish functional coupling.