Several parameters were measured in the growth centers of mandible and long bone of rat pups suckled on dams fed 25% or 6% protein diets. No sex-related differences were found in any parameter either during normal growth or in response to malnutrition. While the total contents of protein, DNA, RNA and calcium were reduced in both tissues, mandible was less affected than long bone, although it appeared to be affected earlier. Moreover, malnutrition reduced the number of cells in the mandible without changing cell size or calcium content. In contrast, long bone appeared to be affected later than mandible, and to be modified differently. In this case, not only cell number, but also cell size and cellular content of calcium were reduced. The results can be interpreted to mean that the critical periods of growth for mandible and long bone are different. Moreover, some suggestion of differences in relative sensitivity to malnutrition of these two hard tissues was also observed.