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The morphological changes that occur during the development of the rumen and ruminal epithelium, and their responses to dietary change, have been reviewed. Particular attention has been paid to the role of chemical factors, including the steam volatile fatty acids (VFAs), as mediators of the adaptive response of the ruminal epithelium. These factors have been investigated in sheep by the combination of rumen cannulation and a biopsy technique. The mitotic index of the ruminal epithelium and the length and width of amen papillae were compared in various regions of the rumen. Significant differences were found in the mitotic indices of the ruminal epithelium and in the length and width of rumen papillae, in lambs fed different diets. There was a rapid, but transitory increase in mitotic index, when the diet of the sheep was changed from one based on roughage to a concentrate-based diet, and when the daily intake of concentrate was increased. Although the VFA proportions also changed as the diet was changed, the mitotic indices could not be directly related to the VFA proportions determined in the rumen liquor. The duration of the phases of the cell cycle was analysed using an isotope labelling method. The duration of the cell cycle was about 24 hours, but this decreased to about 17 hours when cell proliferation was stimulated. The decreased cell cycle time was mainly due to decreases in the duration of the synthesis (S) phase. The cell cycle data and the labelling index were used to calculate the turnover of the basal cell layer, and to estimate turnover times for the epithelium. The mean turnover time for the epithelium was 16.5 days when sheep were fed a roughage based diet, 10.9 days when fed a concentrate diet, and 4.3 days when the sheep were in transition between the roughage and the concentrate diet.