In undisturbed pregnancies of the rat skeletal development coincides with gestational age, and in that case a prolonged gestation is connected with an intrauterine progress of skeletal ossification. In disturbed pregnancy associated with fetal retardation also perinatal ossification is delayed. In the rat this delay can obviously be compensated by a prolonged pregnancy. For estimation of skeletal ossification a simple score has been developed. A total score of all selected perinatal ossification centres (sternebrae, bones of the fore- and hindpaw, caudal vertebrae) was ascertained and compared with a reduced score obtained by scoring the lower part of the body only (caudal vertebrae, hindpaw). With this reduced scoring system the same qualitative relations between control and retarded fetuses and newborns of different gestational age could be established as with the total score. The estimation of the appearance of only one representative ossification centre of the forepaw may give a rapid survey of skeletal development, but seems to be uncertain for detailed characterization.