The report describes a 12-month study to determine the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, (TCDD), an extremely toxic contaminant of the herbicide 2,4,5-T. Male and female rats were intubated twice weekly to provide 0, 0.1 or 1.0 ug/kg/wk of TCDD dissolved in acetone diluted with corn oil. The animals were treated for a total of 28 weeks and held for an additional 12-week recovery period. Body weight observations were made weekly throughout the entire test period. Groups of rats were killed at the end of 2, 4, 8, 16 and 28 weeks of treatment and after 4 and 12 weeks recovery. TCDD was observed to be toxic but not lethal to rats. The extent of the toxic effects, which were manifested by centrilobular fatty change and increased hepatocellular turnover, were confined to the liver and were more severe in male than in female rats.