Intra-oral squamous-cell carcinomas occurred in over 50% of the HMT inbred strain of rats. In the outbred stock from which it was derived the incidence was 5% 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 signficiant 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.