Mitochondria are semi autonomous organelles, with their own genome and transcription/translation systems. Although the regulation of mitochondrial gene expression is fairly well characterized in the animal system, little is known about these processes in plants. We have been studying the expression of ORF25, a mitochondrial open reading frame, in normal male-fertile maize. In all the N lines that we have examined, the ORF25 transcript pattern is similar, except for that in B37N. We have compared ORF25 transcription patterns between B73N and B37N: B73N has one major transcript of 2,300 nucleotides and two minor transcripts of 3,400 and 1,600 nucleotides, while B37N has a single transcript, 3,400 bases long. The ORF25 reading frame and 5' flanking regions have been analyzed by restriction mapping and found to be identical in these lines. Interestingly, the F1 progeny from reciprocal crosses between B73N and B37N have ORF25 transcript patterns identical to B73N. This suggests that the process of mitochondrial transcription is influenced by nuclear factors in normal cytoplasm. This factor(s) appears to be dominant in B73N and the F1 progeny. S1 nuclease analyses have revealed that identical fragments are protected in B73N and the F1 hybrids, indicating that the ORF25 transcripts in the F1 progeny are identical on the 5' ends to those of the parent B73N. This nuclear regulation may be at the level of initiation of transcription or processing of the mtRNA.