Fatty acid synthase (FAS), a key lipogenic enzyme, is expressed in the two major sites of fatty acid production in the body, that is, the liver and the adipose tissue. Surprisingly, the relative contribution of these sites to lipogenesis is highly variable among species. For example, besides the situation in rodents, where liver and fat are equally active, lipogenesis in some mammals such as the pig occurs principally in adipose tissue, whereas in avian species, the liver is the main lipogenic site. We addressed the question concerning the factors determining the site of fatty acid synthesis. We show that the expression of adipocyte determination and differentiation-dependent factor 1/sterol regulatory element-binding protein (ADD-1/SREBP-1) mRNA, but not SREBP-2, is linked to FAS protein content or activity in adipose tissues and livers of pig, chicken, and rabbit. Tissue differences in ADD-1/SREBP-1 mRNA expression between species were paralleled by commensurate variations in the nuclear concentration of SREBP-1 protein. Moreover, overexpression of ADD-1/SREBP-1 by adenoviral gene transfer induces FAS in chicken adipocytes, where lipogenesis is normally low. Conversely, the expression of a dominant negative form of ADD-1/SREBP-1 in pig adipocytes downregulates FAS expression. These results reinforce the role of ADD-1/SREBP-1 as a key regulator of lipogenesis, by extending its importance to nonrodent mammals and birds. Furthermore, they establish that differential expression of ADD-1/SREBP-1 is a key determinant of the site of fatty acid synthesis in the body.-Gondret, F., P. Ferré, and I. Dugail. ADD-1/SREBP-1 is a major determinant of tissue differential lipogenic capacity in mammalian and avian species. J. Lipid Res. 2001. 42: 106;-113.