AIM: To investigate the expression of a-fetoprotein (AFP), a cancer-associated fetal glycoprotein, and its involvement during rat colon development.METHODS: Colons from Sprague-Dawley rat fetuses, young and adult (8 wk old) animals were used in this study. Expression levels of AFP in colons of different development stage were detected by reversetranscriptase PCR (RT-PCR) and West blotting. To identify the cell location of AFP in the developing rat colons, double-immunofluorescent staining was performed using antibodies to specific cell markers and AFP, respectively. RESULTS: The highest levels of AFP mRNA were detected in colons of rats at embryonic day 18.5 (e18.5). Compared to e18.5 d, the AFP expression was significantly decreased during rat development [85% for e20.5, P < 0.05, 58% for postnatal day 0.5 (P0.5), P < 0.05, 37% for P7, P < 0.05, 24% for P14, P < 0.05, and 11% for P21, P < 0.05] and undetected in adult rats. Only the 72-kDa isoform of AFP was detected by West blotting, the expression patt was similar to AFP mRNA and conformed to the results of mRNA expression. The AFP positive staining was identical to different distribution patts in fetuses, young and adult animals and positive staining for both AFP and vimentin was overlapped in mesenchymal cells at each stage tested. CONCLUSION: This study has for the first time This study has for the first time demonstrated that AFP is localized in the mesenchyme of rat colon from the embryo to the weaning stage by immunofluorescence and presents 72-kDa isoform in the developing rat colons by West blotting. The dynamic expression of AFP in the various developmental stages of the colon indicates that AFP might be involved in many aspects of colon development.