Mitogen-activated protein kinases (MAPKs) are components of a kinase module that plays a central role in the transduction of diverse extracellular stimuli, including mitogens, specific differentiation and developmental signals and stress treatments. This shows that reversible protein phosphorylation cascades play a pivotal role in signal transduction in animal cells and yeast, particularly the entry into mitosis of arrested cells. Homologues of MAPKs have been found and cloned in various plant species, but there have been no data about their in situ localization at the subcellular level and their expression in plant cells so far. In the present paper we report the first data on the ultrastructural in situ localization of MAPK and their mRNAs in various plant cells. Proliferating and quiescent meristematic plant cells were studied to evaluate whether changes in MAPK presence, distribution and expression accompany the entry into proliferation of dormant cells. Moreover, MAPK localization was analyzed in vacuolate microspores. Polyclonal antibodies against the deduced MAPK from the tobacco Ntf6 clone were able to recognize homologue epitopes by immunocytochemical techniques in the cell types studied. The pattern of protein distribution is similar in all the cases studied: it is localized in the cytoplasm and in the nucleus, mainly in the interchromatin region. The quantitative study of the density showed that MAPK labelling is more abundant in cycling than in quiescent cells, also suggesting that, in plants, MAPK pathways might play a role in cell proliferation. RNA probes for conserved regions of the catalytic domain of plant MAPK homologue genes were used to study MAPK expression in those plant cells. In situ hybridization (ISH) showed the presence of MAPK transcripts in the three plant cell types studied, but levels were very low in quiescent cells compared to those in cycling cells. The quantification of labelling density of ISH signals strongly suggests a higher level of MAPK expression in proliferating cells, but also some basal messenger presence and/or expression in the quiescent ones. Immunogold and ISH results show the presence and distribution of MAPK proteins and mRNAs in vacuolate microspores. This represents a very dynamic stage during pollen development in which the cell nucleus is being prepared for an asymmetrical mitotic division, giving rise to both the generative and the vegetative nuclei of the bicellular pollen grain. Taken together, the data indicate a role played by MAPK in the re-entry into proliferation in plant cells.