Total tooth length was measured in the first lower molar of two short-faced strains of rats (Sprague-Dawley and Buffalo) and two long-faced strains (BN and GRL). Tooth length was significantly longer in the long-faced than in the short-faced strains. The same correlation had previously been found in dogs, baboons and human populations, as well as in rats in which facial shortening was obtained experimentally by septum removal. The data suggest that the same mechanism is responsible for this relationship in all groups.