Alterations in protein synthesis in rat liver cells were examined following in vitro and in vivo exposure to 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD). Primary cultured rat liver parenchymal cells were exposed to 1 nM TCDD for 23 and 47 hr. Synthesis of two proteins with molecular weights ( M r) of 26,000 and 39,000 (designated 26k-P and 39k-P, respectively), other than cytochrome P450, was increased markedly in the cells. These proteins did not have the same antigens as cytochromes P450IA1 and P450IA2. Synthesis of three proteins with M rs of 24,000, 25,000 and 29,000, respectively, was decreased by TCDD. TCDD was administered to rats at a dose of 100 μg/kg body weight. The amount of five proteins (two proteins with M r of 26,000, one of 36,000 and two of 39,000) was increased in TCDD-treated rat liver. However, the proteins increased in vivo by TCDD were distinguishable from 26k-P and 39k-P by two dimensional gel electrophoresis.