In vivo lipogenesis was estimated in liver and carcass of male chickens selected for leanness (LL) or fatness (FL) by use of tritiated water. Effects of nutritional state and of a high fat diet (90 g/kg) were examined. Different classes of liver lipids were also measured. Fed birds exhibited enhanced hepatic lipogenesis, which was more pronounced in FL than in LL birds (+73%, mean of three experiments). Extrahepatic lipogenesis was poorly influenced by nutritional state in LL birds and not modified in FL ones. The high fat diet induced a fall in liver triacylglycerol and reduced de novo lipogenesis more drastically in liver than in carcass of both lines. Reduction of carcass lipogenesis was less pronounced in FL than in LL. Starvation led to a fall in liver triacylglycerols, nonesterified fatty acids and free cholesterol. The fat diet had similar but less pronounced effects. In most circumstances FL chickens exhibited a higher liver triacylglycerol content than LL ones. Comparison of fed and refed birds 100 min after the beginning of refeeding showed that de novo lipogenesis did not reach a plateau and differences between lines were still not significant.