By means of roentgenographic cephalometry and quantitative genetic analysis, the relative contribution of the genetic and environmental components to total variance of body weight and overall craniofacial size was shown to vary with age. The genetic component of variance significantly increased until 80 days of age. Inversely, the maternal component of variance showed a high value during the early stage of postnatal growth and gradually decreased thereafter to a very small amount by day 80. Thus it appeared that the genetic effect became larger with age of the rat and the maternal effects diminished. The environmental component of variance did not change much over the course of the experiment. We thus conclude that genetic effect contributed the change of ontogenetic variation of craniofacial complex through all experimental periods and that maternal effect contributed to it at early growth stage of the craniofacial complex.