A method for the determination of the total muscle mass in small experimental animals is presented. Muscle proteins were quantified in the 1 M LiCl-soluble and distilled water-insoluble fraction of the eyeless, brainless, eviscerated and skinned carcass, as compared with a striated muscle sample from the same animal used as standard and processed in the same way as the whole carcass. The non-muscular tissues left in the carcass do not interfere with the final results. The total muscle mass in adult rats was estimated as 42.0 ± 2.8 % of the ln vivo weight.