The role of reflux oesophagitis and acid in the development of oesophageal columnar epithelium was investigated in the rat. Twenty-four animals underwent total gastrectomy and oesophagojejunostomy, inducing reflux of duodenal contents into the oesophagus, and were divided into two groups: an 'acid' group, which drank syrup water with hydrochloric acid (pH 1.8), and a 'non-acid' group, which drank only syrup water (pH 6.5). Four rats were killed at 4, 8 and 12 weeks after surgery. Reflux oesophagitis was found in the lower portion of the oesophagus in all cases. Columnar epithelium was observed in the oesophagus of one rat in the non-acid group at week 8 and of three in the acid group at week 12. These results indicate that the development of columnar epithelium of the oesophagus is preceded by reflux oesophagitis and that acidic conditions may promote this change. Columnar epithelialization may result from metaplasia of squamous epithelium as well as from upward extension of columnar epithelium.