The expression of specific mitochondrial, chloroplast and nuclear genes has been investigated in leaves of 7-d-old light-grown wheat (Triticum aestivum cv. Maris Dove). In the wheat leaf there is a spatial separation of a temporal sequence of development from the basal meristem to the distal mature, photosynthetically competent cells. This sequence of cellular differentiation is paralleled by a functional differentiation in which the energy supply changes from oxidative phosphorylation in the non-green meristematic cells to a combined dependence on oxidative and photophosphorylation in the photosynthesizing cells. The changes in copy number per cell and expression of mitochondrial genes have been investigated in successive sections of the wheat leaf using quantitative DNA-DNA and DNA-RNA filter and protein-binding techniques. The abundance of specific mitochondrial genes (cox II.cob andatp A) per cell was found to decrease between five- and tenfold within the basal (1 cm) section of the leaf and then remain constant to the distal tip. The relative abundances of specific mitochondrial transcripts (cox I,cox II,cob andatp A) were found to decrease in successive sections from the basal meristem to the distal tip (from a relative value of 100% to 5-40%). In contrast, transcripts of chloroplast genes and nuclear genes encoding chloroplast polypeptides (psb A,rbc L andrbc S) were found to increase steadily in progressive leaf sections (from a relative value of 0-2% to 100%). The steady-state level of the α-subunit of the mitochondrial F1 ATPase was found to remain constant along the length of the leaf. Possible sites at which the regulation of organellar gene expression is coordinated during the development of photosynthetic competence within the wheat leaf are discussed.