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As far as we know, a caries-susceptible colony of rats has not hitherto been reared in this country. We were fortunate to receive a few animals from the stock maintained at the Harvard School of Dental Medicine. This colony has been produced over a number of years and is susceptible to the development of dental caries (Sognnaes, 1948; Shaw, 1950). All our rats were the descendants of four pairs of Harvard animals and have been multiplied by random mating. Brother-sister mating was not used. The colony is therefore more properly described as a caries-susceptible ' stock' rather than a 'strain'.In considering experimental rat caries it is important to distinguish between the types of the disease known to occur in this animal. Hoppert, Webber & Canniff (1932) found that a diet containing coarsely ground maize (HWC diet) leads to the production of dental caries in their rats. If the maize was finely milled to pass a 60-mesh sieve and then incorporated in the diet no caries resulted. Cox, Dodds, Levin & Hodge (1948) showed that, after an initial period on the HWC diet, a high intake of sucrose (67 yo of the diet) causes an increased rate of progression of caries, but that a high level of sugar in the diet of animals that had not previously received the HWC diet does not lead to the production of caries. As a result of these experiments they concluded that, in their animals, caries is initiated by the fracturing of the enamel owing to the impaction of coarse maize particles in the occlusal fissures of the rat molars. King (1935) had previously put forward this view, but he studied only relatively few rats. Cox et al.

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