Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activase catalyzes the activation of RuBisCO in vivo. Two full length cDNAs designated as OsrcaA1 and OsrcaA2 encoding two RuBisCO activase isoforms of 47 and 43 kDa, respectively, have been cloned and characterized. The two isoforms were 99% identical, the 47 kDa isoform having an additional 33 amino acids and a 5 amino acid substitution at the carboxyl terminus. The deduced amino acid sequences of OsrcaA1 and OsrcaA2 showed 73-89% identity with RuBisCO activase from other higher plants. Two highly conserved ATP binding sites were identified. The Osrca mRNAs, and the RuBisCO activase proteins of 43 and 47 kDa were specifically detected in leaf, but not in root or etiolated seedling tissues. During leaf development, the abundance of Osrca mRNAs increased from the 7th to the 3rd leaf, and reached a maximum in the 2nd leaf, although the amounts of the 43 and 47 kDa RuBisCO activase remained almost unchanged among the six leaves, indicating the involvement of post-transcription control in the regulation of RuBisCO activase expression in rice. The co-immunoprecipitation of RuBisCO LSU and SSU with RuBisCO activase suggests that RuBisCO activase interacts with RuBisCO in vivo.