We have studied the genomic structure and evolutionary pattern of immunoglobulin kappa deleting element (KDE) and three kappa enhancers (KE5', KE3'P, and KE3'D) in eleven mammalian genomic sequences. Our results show that the relative positions and the genomic organization of the KDE and the kappa enhancers are conserved in all mammals studied and have not been affected by the local rearrangements in the immunoglobulin kappa (IGK) light chain locus over a long evolutionary time ( approximately 120 million years of mammalian evolution). Our observations suggest that the sequence motifs in these regulatory elements have been conserved by purifying selection to achieve proper regulation of the expression of the IGK light chain genes. The conservation of the three enhancers in all mammals indicates that these species may use similar mechanisms to regulate IGK gene expression. However, some activities of the IGK enhancers might have evolved in the eutherian lineage. The presence of the three IGK enhancers, KDE, and other recombining elements (REs) in all mammals (including platypus) suggest that these genomic elements were in place before the mammalian radiation.