Correlations of 12 skull measurements in two European mammal species-the common polecat (Mustela putorius L.) and muskrat (Ondatra zibethicus L.) were examined. Correlation connections between different skull measurements were plotted on Czekanowski's diagrams and the correlation structures of the two species were compared by two methods: Terentjev's correlation pleiads and Vyhandu's one. Considerable differences were found in the correlation structures of the skull. The indicator-characters of the skull: condylobasal length (CbL) and diastema length (DL)-exhibit different degrees of connections with the other skull measurements in both of these species. The skull of the muskrat is distinguished by a slight degree sex dimorphism in dimensions which increases with the animal's age. Intensive increase of the majority of its dimensions ends in the second calendar year of the animal's life. Age changes of skull dimensions of the muskrat are also reflected in changes in correlation structures corresponding to periods of isometric and allometric growth. It is possible that these little known statistical methods may have a practical application in medicine.