trans-Splicing is the biological reaction that generates a mature mRNA from separate strands of pre-mRNAs. Previously, we reported that the trans-splicing between the two Sp1 pre-mRNA strands produced an mRNA with the exon 3-2-3 alignment in human HepG2 cells. Here we describe the rat counterpart as well as a newly identified variant with the exon 3-3 alignment in cultured rat cells. A qualitative evaluation of such alignments in poly(A)(+) RNA-rich preparation showed that both alignments arose from trans-splicing rather than circularization of a single strand. The identification of the trans-spliced products in both rat and human raises the possibility that trans-splicing on Sp1 pre-mRNA is rather common to mammals. It was observed that the level of the trans-spliced variants varies in different rat organs.