Mitogen-activated protein kinase (MAPK) cascades consist of members of three families of protein kinases: the MAPK family, the MAPK kinase family, and the MAPK kinase kinase (MAPKKK) family. Some of these cascades have been shown to play central roles in the transmission of signals that control various cellular processes including cell proliferation. Protein kinase NPK1 is a structural and functional tobacco homologue of MAPKKK, but its physiological function is yet unknown. In the present study, we have investigated sites of expression of the NPK1 gene in a tobacco plant and developmental and physiological controls of this expression. After germination, expression of NPK1 was first detected in tips of a radicle and cotyledons, then in shoot and root apical meristems, surrounding tissues of the apical meristems, primordia of lateral roots, and young developing organs. No expression was, however, observed in mature organs. Incubation of discs from mature leaves of tobacco with both auxin and cytokinin induced NPK1 expression before the division of cells. It was also induced at early stages of the development of primordia of lateral roots and adventitious roots. Thus, NPK1 expression appears to be tightly correlated with cell division or division competence. Even when an inhibitor of DNA synthesis was added during the germination or the induction of lateral roots by auxin, NPK1 expression was detected. These results showed that the NPK1 expression precedes DNA replication. We propose that NPK1 participates in a process involving the division of plant cells.