Five experiments were reported, involving a total of over 350 mice, in feeding trials concerned mainly with growth rates and feed utilization. One experiment involved gestation-lactation performance of mice fed rapeseed oilmeal as a ration component.Proportions of rapeseed oilmeal ranged from 17 to 50 per cent of the diet. The feeding of iodinated casein or of potassium iodide failed to counteract the growth-depressant factor in rapeseed oilmeal. Aureomycin as a feed supplement was ineffective against the factor.Growth, tissue composition and lactation data confirmed the deleterious nature of high levels of rapeseed oilmeal in the ration and provided evidence that species and sex differences in response to the rapeseed oilmeal factor exist. Males appeared to be more severely affected than females. Mice did not respond to iodine and to iodinated casein as has been reported for chickens and turkeys.