The boundary of transcription units is defined by the sites at which RNA synthesis is initiated and terminated. Much is known about initiation of RNA synthesis and elongation of transcription, but transcription termination is far less well defined in eukaryotic system. To identify the transcription termination elements in mouse gastrin gene, we examined RNA transcripts after stable transfection of gastrin expression plasmids into the NIH3T3 cell line. GT-repeat region at the 3`` flanking sequence of mouse gastrin gene acted as a transcription terminator. The polyadenylation signal was not required for the transcription termination of mouse gastrin gene. When the GT-repeat unit was deleted from its site, the effect of termination disappeared. Further experiment, using serial deletion mutants, revealed that the 56-38 nucleotides upstream region from GT-repeat unit also participated in transcription termination. Studies with purified RNA polymerase II and dC-tailed template bearing GT-repeat unit identified that this sequence was not recognized as intrinsic termination signal by RNA polymerase II. The suggestion that recognition for termination may involve nuclear factor(s) received substantial support from the gel retardation studies. In DNA-mobility shift assay, nuclear factor bound sequence-specifically at the GT-repeat unit. We suggest possible model for RNA polymerase II transcription termination in mouse gastrin gene. The terminator structure was composed of three elements. The upstream region might be recognized as a pause site by the RNA polymerase II. So that its ability to elongate efficiently would be impaired. And DNA-binding factor(s) bound to GT-repeat unit and induced an abnormal DNA structure, like as DNA bend or Z-DNA. The abnormal DNA structure might function as a blockage of transcribing complex processivity.