Calcium-dependent protein kinases (CDPKs) are found in various subcellular localizations, which suggests that this family of serine/threonine kinases may be involved in multiple signal transduction pathways. CDPKs are believed to be involved in the response of plants to low temperatures, but the precise role in the signal transduction pathway is largely unknown. Previous reports described changes in CDPKs' mRNA levels in response to cold treatment, but whether these changes are accompanied by increases in protein level and/or kinase activities is unknown. In the present study, we identify in rice (Oryza sativa L. cv Don Juan) plants a 56-kD membrane-bound CDPK that is activated in response to cold treatment. Immunoblot analysis of the enzyme preparations from control and cold-treated plants showed that the kinase level was similar in both preparations. However, both kinase and autophosphorylating activities of the enzyme prepared from cold-treated plants were significantly higher than that obtained from control plants. The activation of the CDPK is detected after 12 to 18 h of cold treatment, which indicates that the kinase does not participate in the initial response to low temperature but in the adaptative process to adverse conditions. To our knowledge, this is the first demonstration of a CDPK that is posttranscriptionally activated in response to low temperature.