Endomyocardial fibrosis in rats treated with N-nitrosomorpholine.

Endomyocardial fibrosis was observed after long lag periods in male Sprague-Dawley rats treated for 1-14 weeks with the carcinogen N-nitrosomorpholine. The fibrosis developed predominantly in the left ventricle. It occurred during 29-78 weeks after withdrawal of the carcinogen in 5% and 79-108 weeks after withdrawal in 20% of the experimental animals, but was never observed in controls of the same age. We suggest that endomyocardial fibrosis was induced by a direct effect of the carcinogen on the fibroblasts of the endomyocardium.