Abstract: The purified chloroplast FDPase was activated by preiaeubation with DTT or NADH, which was then removed by Sephadex G-25 column and the activated enzyme was obtained by elution. The effect of the pH value of preincubating medium and some reducing or oxdizing agents on FDPase activation as well as the time course of activation during preincubation were investigated. It was found that strong reducing agents such as dithiothreitol (DTT), NADH or Na2S2O4 could all activate chloroplast FDPase, whereas oxidizing agents such as cystine, NAD+, FMN (oxidized form) and Vitamin Ks could all inhibit the activity of the activated FDPase almost completely. However, the degree of activation was strictly dependent on pH of the preincubation medium, the activety at pH 7.8 was 4.7 times higher than that at pH 5.5. The course of activation was rather quick, after only 30 s preincubation with DTT, the activity of FDPase was up to one half of its maximum value. On gel electrophoresis, the absorbance profile of strips of the activated enzyme was different from that of non-activated one. As a result, the two strips of the latter seemed to be combined into a sharp strip in the activated one. This phenomenon might be the cause of the activation of this enzyme. The present study has demonstrated that chloroplast FDPase can directly be regulated by certain physiological redox effectors. This will perhaps contribute to a better understanding of the regulatory mechanism of light activation and dark inactivation of FDPase within chloroplasts.