The energy supply for eukaryotic ciliary and flagellar movement is thought to be maintained by ATP-regenerating enzymes such as adenylate kinase, creatine kinase and arginine kinase. In this study, the energy-supplying system for the ciliary movement of Paramecium caudatum was examined. Arginine kinase and adenylate kinase activities were detected in the cilia. To demonstrate that phosphoarginine satisfactorily supplies high-energy phosphate compounds into the narrow ciliary space, we prepared an intact ciliated cortical sheet from live Paramecium caudatum. These cortical sheets, with an intact ciliary membrane, produced a half-closed system in which each cilium was covered with a ciliary membrane with an opening to the cell body. Ciliary beating on the intact cortical sheets was induced by perfusing not only ATP but also ADP. Addition of phosphoarginine (0.2 mmol l(-1)) increased the beat frequency. A further increase in beat frequency was observed in 0.4 mmol l(-1) phosphoarginine, and this was enhanced when the cilia were reactivated with relatively low concentrations of ATP. We have demonstrated that phosphoarginine supplies energy as a 'phosphagen' for ciliary beating in Paramecium caudatum, suggesting that phosphoarginine functions not only as a reservoir of energy but also as a transporter of energy in these continuously energy-consuming circumstances. http://www.biologists.com/JEB/movies/jeb3123.html