NZR/Gd is a new inbred strain of albino rats subject to epithelial mesotheliomas of the heart and also to hepatic glycogen storage disease. The age-specific incidences of neoplasms in 650 untreated NZR rats are tabulated by sex and histopathologic type. In males the most common neoplasms were artio-caval mesotheliomas (12 per cent.) and lung adenomas (10 per cent.), while in females most were pituitary adenomas (23 per cent.), lung adenomas (10 per cent.) or tumours of breast (7 per cent.), heart (4 per cent.) or liver or uterine cervix (3 per cent.). Uncommon tumours (less than 2 per cent.) were pancreas islet cell adenomas, lymphoma, hemangioma, fibroma or tumours of kidney, ovary or bile ducts. The detailed histopathology of the artio-caval tumours reported earlier was confirmed, but it now appears there is a male preponderance (4 : 1), and larger tumours showed their malignancy by invasion of adjacent structures and occasionally by distant metastasis. In males the primary site was evenly divided between right atrium and inferior vena cava. Metastasis of tumours of all types was uncommon, but multiple primary tumours were frequently found, especially in the endocrine system of female rats. Almost one-third of the tumours were associated with at least one other neoplasm arising in a different tissue. Overall tumour statistics in NZR/Gd rats were quite similar to other inbred strains for which quantitative reports are available, and the virtual absence of other diseases and long life-span (up to 1250 days) indicated no serious effect of the hepatic glycogen storage disease on survival. The NZR rats seem to be unique in producing atrio-caval mesotheliomas, and they have a much higher incidence of alveolar lung adenomas (10 per cent.) tham other reported strains.