The spontaneous rate of tumours in the laboratory rat: strain Chbb: THOM (SPF).

The aim of this investigation was to examine the time relationship of the rate of development of spontaneous tumours in the rat, strain CHBB: THOM (SPF). The rats were divided into 6 groups, each of which consisted of 100 male and 100 female animals. The first group was killed at the age of 15 months, and the last group when aged 2 1/2 years. Thus the time interval between sacrificing the individual groups was 3 months. Spontaneous tumours seldom appear in animals up to 15 months of age except in the testes, pituitary and mammary glands. Apart from these organs tumours develop mainly in the lung, lymph nodes, thymus, adrenal cortex, thyroid, ovary, uterus, skin and in the brain. Benign tumours predominate. Malignant tumours with metastases are rare. The incidence of tumour formation increases after the age 21 months. Statistically significant increases can only be shown in a few organs.