Histologic, ultrastructural and nick end labeling studies were made of the process of apoptosis in the bulbus cordis of rat embryos. Apoptosis was observed between the 14th and 16th days of gestational age, at which time the bulbus cordis undergoes extensive remodeling. Three types of mesenchymal cells were identified in this region: cells undergoing apoptosis, cells engaging in the phagocytic removal of apoptotic cells, and cells not involved in either of these two processes. Fragmentation of DNA, demonstrated by nick end labeling, was found only in the apoptotic cells. The combined use of morphologic and labeling techniques is extremely useful in the evaluation of the contribution of apoptosis to cardiac morphogenesis.