Transcription factor NF-kappa B represents a family of closely related homo- and heterodimeric factors. The most abundant form of NF-kappa B is the p50/p65 heterodimer. We determined the complete genomic structure of the human gene and a partial structure of the mouse gene encoding p65. The human gene consists of ten exons and spans about 8.1 kbp of DNA. The exon-intron organization in the rel homology domain (exons 2 to 7) is conserved when compared to human and turkey c-rel, strengthening the evolutionary relationship between p65 and c-rel. The lengths of the corresponding introns 5 and 6 in the human and mouse p65 genes are not conserved. However, a surprisingly high degree of conservation of intron sequences was observed between both species. We show that the naturally occurring shorter variant of p65 (p65 delta) can be generated by alternative splicing of intron 6, not only in humans but also in mouse. In addition, the existence of another, as yet unknown splice variant of p65 is predicted.