The relative feed intake, feed efficiency and carcass characteristics of growing pigs and mice were studied to determine the value of the mouse as a pilot animal in pig nutrition research. Fifteen rations were fed in two experiments to 120 pigs and 120 mice. They were composed of equal parts of wheat and barley, plus one of three levels each of oat hulls, alfalfa meal, wheat bran, cellulose, and corn-cob meal to produce mixtures containing an estimated 62, 65, and 68 per cent of total digestible nutrients. With pigs, data were obtained on feed and growth from 100 to 200 pounds live weight; carcass measurements were made on the slaughtered barrows at 200 pounds. Feed and growth records of mice were kept for 14 days commencing at weaning; the carcasses were then analysed for water, fat, protein and ash.The relative response of the two species is indicated by the following correlation coefficients: feed consumption (0.67); digestibility of dry matter (0.96); digestibility of energy (0.87); consumption of digestible energy (0.79); rate of gain (0.63), and per cent fat in mouse carcasses and back fat thickness in the barrows (0.70). The correlation between per cent fat in mouse carcasses and the R.O.P. score for barrows was −0.67.The mean ratio of swine and mice was 581:1 for physiological body weight (kg.0.75), 693:1 and 654:1 for consumption of dry matter, and digestible energy, respectively, reflecting a relatively greater feed capacity of pigs and a higher energy digestibility by mice. The corresponding ratios for the effects of changes in ration composition were 369:1, 331:1, and 462:1, indicating that the pigs were better equipped to cope with bulky rations. It was suggested that mice were more efficient in conversion of feed to body weight gain because in pigs a greater part of the gain was fat.