Summary.-Intra-oral squamous-cell carcinomas occurred in over 50O0 of the HMT inbred strain of rats. In the outbred stock from which it was derived the incidence was 5O% or less, both when inbreeding was begun and after the inbred strain was fully established. Various factors in food and husbandry which might have irritated the oral mucosa were investigated, but there was no significant evidence that they played any part in the high incidence of mouth tumours.It is concluded that there must have been an accidental selection during inbreeding in favour of rats which had an inherited tendency to develop squamous -cell carcinoma of the mouth.There are a number of similarities between the rat tumour and intra-oral squamous-cell carcinoma in man and it is suggested that the rat tumour could be used as a model of the human disease.A BREEDING NUCLEUS of the "Alderley Park (Strain 1)" SPF outbred albino rats (Paget & Lemon, 1965) Leonard, personal communication, 1974).The first evidence of a high incidence of squamous-cell carcinoma of the mouth was in rats of the 12th-l 5th generation of inbreeding, which came froml anl experiment involving localized irradiation of the testes (Hulse, 1977). These rats were born 1970-1971, and during the latter part of 1973 a surprisingly large number developed intra-oral carcinoma of the mouth. Only males were observed at that time, and many had been irradiated, albeit well away from the mouth, so a formal investigation was necessary to find the exact incidence in both sexes. A number of factors in the rats' environment which might possibly have accentuated any tendency to develop mouth tumours were also examined. METHOD OF iNVESTIGATIONThe rats' cages, their food and their driniking water were thought to be possible sources of irritants to their oral mucosa. The incidence of mouth tumours wxas also studied in a group of rats imported from the parent stock.Cages.-The galvanized-wire cages which wxve w%vere using were old. and, as the galvanizing was inclined to come off, it was conceiv-* Pr'eseint adclress: H imitingdon Researcht Centre, Ht1itltiIg(olon, Cambri(dgesli1il(r.Requiests for reprints: Dr E. V. Hulse.