Aflatoxin B1 was fed at 2 ppm in the diet to a group of pregnant F344 rats from the time of conception; it was then fed to their offspring until death. This diet was also given to another group of rats 6-7 weeks old for comparison. The survival time of male rats was significantly shorter than that of the female rats of both groups. However, the survival times of rats of the same sex in both groups did not differ significantly. The major causes of death were hepatic neoplasms with matastases, although some early deaths occurred before neoplasms developed. Most deaths were from a malignant hemorrhagic liver tumor, histologically diagnosed as a hemangiosarcoma, which caused rupture and hemorrhage into the peritoneal cavity or metastases to the lungs. These hemangiosarcomas were readily transplantable and did not produce alpha-fetoprotein. Ultrastructurally, they were composed of poorly differentiated cells resembling endothelial cells. Nodules of hyperplasia induced by aflatoxin B1 sometimes grew large (greater than 1.5 cm), and 2 were transplanted. Approximately 20% of the rats had colon tumors; a few rats had tumors of the kidney, oral cavity, and hematopoietic system.