The development of the sternum was studied in normal and thalidomide‐treated rat embryos and fetuses. Morphologically, the sternal abnormalities included: crowding of the sternocostal articulations, absence of sternebrae, “scrambled” sternebrae, paired sternebrae, torsion of the sternum, partially cleft sternums, and completely bifurcated sternums. These malformations were related to an abnormal rib pattern and to a delayed growth rate of the ribs and sternebral primordia. Histochemically, the distribution and relative amounts of glycogen, alkaline phosphatase, calcium, and ribonucleic acid were studied. In cartilage the degradation of glycogen and the activity of alkaline phosphatase were influenced by thalidomide. This resulted in increased storage of glycogen and a reduction or absence of alkaline phosphatase activity. These changes were reflected in the rate of deposition of the cartilage matrix, and were most notable at the hypertrophic cartilage stage. Calcification of the cartilage was delayed, indicating the retardation in matrix deposition. No changes were observed in the staining pattern of ribonucleic acid. The deposition and calcification of endochondral bone was delayed, possibly caused by the delay in chondrification rather than by a direct effect on the ossification mechanism. No changes were observed in the development of the intramembranous, subperiosteal bone.