Dentinal molar fissure caries in Liverpool albino rats, produced by initial exposure to dietary sucrose, could be significantly reversed by subsequent exposure to a xylitol-supplemented starch diet (X-S St). An unsupplemented starch diet had no significant effect on caries under the same conditions. Alternate feeding of sucrose and X-S St produced a sequential progression and regression of caries, respectively. Continued exposure to X-S St produced a successive regression in caries. The regression in caries can be explained in terms of the increased rate of salivary flow and the attendent rise in pH and calcium concentration that occur when xylitol is introduced into the mouth. Both of these changes should encourage re-mineralisation of a reversible lesion below a temporarily non-acidogenic plaque. Under these conditions xylitol is not merely a bland, non-cariogenic agent but is genuinely exerting a therapeutic action against caries.