Two novel rice (Oryza sativa L.) protein kinase (PK) genes have been isolated. OsMSRPK1 and OsMSURPK2, which most likely exist as single-copy genes in the rice genome, encode 693 and 503 amino acids polypeptide, respectively, and have the serine/threonine kinase domain of cyclin dependent protein kinase (OsMSRPK1), or the serine/threonine kinase domain and NAF domain (OsMSURPK2). Steady-state mRNA analyses of these PKs, with constitutive expression in the leaves of two-week-old seedlings, revealed that OsMSRPK1 is up-regulated upon exposure to environmental stresses, whereas OsMSURPK2 is down-regulated by these same stresses. Furthermore, the two PKs are developmentally regulated in both young and mature rice plants, including in the panicles. These results strongly suggest that the genes have roles in both plant development and in their defense/stress-signaling pathways.