In a long-term study in Han:WIST rats, 320 males and females each, the body weights were registered over the whole lifespan. After the natural end of the animals all pathological findings were recorded. The influences from pituitary tumours, progressive glomerulonephrosis, uterine adenocarcinomas and mammary fibroadenomas on the average body weight development and the final body weight were calculated and statistically evaluated.