A new autosomal recessive mutation in the mouse, little (lit), has been shown to be located on Chromosome 6. The mutation in the homozygous state causes ateliotic dwarfism that is first detected at 15 days of age by decreased body weight. Long bone lengths are significantly reduced. Skull width, however, is not affected. Female little mice are fully fertile; they may lose their first litters. Although most of the little males sire one or two litters, they rarely sire a third litter. Analysis of pituitary extracts electrophoresed on acrylamide gels reveal a significant reduction of the two anterior pituitary hormones, GH and PRL, in both male and female little mice. Because the little mouse shares a number of similarities with the human ateliotic dwarfism, isolated growth hormones deficiency type I, it may be a useful animal model for this inherited human growth disorder.