Mitogen-activated-protein (MAP) kinases are components of signal transduction pathways which respond to a variety of stimuli in different organisms. In quiescent mammalian cells, the reactivation of cell division induced by different mitogenic signals is mediated by the rapid phosphorylation and activation of MAP kinases. We have investigated whether a similar situation occurs in plants, arresting tobacco (Nicotiana tabacum L.) cells in the G1 phase of the cell cycle by phosphate starvation, and then inducing them to re-enter the cell cycle by refeeding with phosphate. The transient activation of a kinase activity with the characteristics of a MAP kinase was observed during the first hour after refeeding, when the cells were still in G1. Using myelin basic protein (MBP) as substrate, an increase in this phosphorylating activity, with a molecular mass of approximately 45 kDa, was detected in cell extracts between 35 and 55 min after induction, in in-gel phosphorylation assays and after immunoprecipitation with anti-MAP kinase antibodies. The specificity of the antibodies against recombinant tobacco MAP kinases suggested that the MAP kinase p45ntf4 was responsible for the observed activity. These data provide experimental evidence for the activation in vivo of a plant MAP kinase, possibly mediating the reactivation of cell division in G1-arrested cells.