Rhythm is an important dynamic behavior in biological systems. We have been studying oscillatory reactions of enzymes induced by gradual entry of substances through semipermeable membrane. Not only enzymes but also a few species of substance of living system have been elucidated to cause oscillatory reaction. Here we present the oscillatory reaction by chondroitin sulfate in a system of gradual entry of calcium ion. Introducing calcium ion through dialysis membrane into chondroitin sulfate solution induces an oscillation of free calcium ion concentration in chondroitin sulfate solution. Simultaneously, it is elucidated that oscillation of conformation occurs with permeation of calcium ion. In both measurements, oscillations with 25 It period are obtained. The phases of oscillation, however, differ slightly from each other. From these results, it is suggested that autocatalysis exerts in the contraction of chondroitin sulfate conformation. These phenomena are very intriguing for elucidating oscillation in living system. (C) 2004 Elsevier Inc. All rights reserved.