We have used RNA in situ hybridization to analyze the expression of transcripts encoding cytochrome c in different tissues and organs of sunflower (Helianthus annuus). Although northern-blot hybridization experiments indicate that the relative abundance of transcripts does not vary greatly, we have detected important changes in localization during flower development. Enhanced expression is observed in floral meristems as soon as they are discernible from the central portion of the capitulum containing the inflorescence meristem. As flowers develop, labeling is observed in all developing floral organ primordia. Later in development, expression in petals is reduced, and only the central portion of the flower becomes labeled. During the process of stamen formation, hybridization signals were obtained mainly in anthers. Less developed flowers at this stage showed expression through the archesporial tissue. During meiosis, the label was observed mainly in tapetal cells. Specific expression patterns, similar to those obtained for sunflower, were observed when Arabidopsis flowers were analyzed with a homologous cytochrome c probe. Specific patterns of expression were also observed in young sunflower roots. In this case, enhanced expression was detected in developing endodermis and pericycle and in protoxylem initials. We conclude that cell-specific mechanisms operate to regulate the abundance of cytochrome c encoding transcripts in different plant tissues. The overlap between the expression patterns of the nuclear encoded cytochrome c gene and some mitochondrial genes suggests the existence of coordinated mechanisms of expression.