After a critical review of recent research on the whole body chemical composition of laboratory animals, especially that of the mouse, the authors present data on the body composition of a mouse line of different body condition score, varying from normal to double-muscled ("culard", "compact") type. One hundred male and 100 female mice of different compact categories were measured in vivo, using total body electrical conductivity (TOBEC method), then by direct chemical analysis performed on day 77 of life. In male mice, body weight and protein content were found to increase while fat content decreased parallel to the increase in phenotypic condition score. Female mice showed a rather constant fat, protein and ash content in the dry matter of the body. The precision and validity of TOBEC data under 40 grams of body weight proved to be insufficient.