Secondary metabolites are known to be involved in the interaction between plants and pathogens (1). Some of them are highly suspected to contribute to resistance against fungi, such as stilbenes (2, 3). Stilbenes are the main phytoalexins in grapevine (4). Downy mildew (Plasmopara viticola, Oomycetes) causes huge damage in vineyards. So the creation of new varieties is expected to reduce phytochemical applications. These varieties will be obtained by the pyramiding of complementary resistance genes. Thereby, a better knowledge of the effect of these genes on the plant response is important to acquire. Two populations derived from the American species Muscadinia rotundifolia and previously phenotyped for their resistance to downy mildew were studied for their ability to synthesize stilbenes. The first population called 41528 possesses two major resistance genes: Rpv1 which slows down the progression of the pathogen in the leaves and Rpv2 which blocks it at a very early stage. The second population 41524 contains only Rpv1. Mocked and inoculated leaves were studied on about 100 genotypes of each the two populations. One to three plants were studied per genotype. Leaves of level 5 (level 1 corresponded to the first spread leaf at the top of the stem) and 6 were harvested. Level 6 was inoculated with Plasmopara viticola. Leaf discs were sampled at 4 days post infection (dpi) and 5 dpi then extracted with MeOH at 60°C. Stilbenes (piceid cis and trans, resveratrol, viniferins delta and epsilon, pterostilbene) were analyzed with HPLC-DAD and in some cases with HPLC-mass spectrometry (MS-MS). The sum of 6 stilbenes and pterostilbene contents were highly correlated between 4 and 5 dpi (R2 respectively of 0.7 and 0.84); showing a linear activity of stilbene synthase and O-methyl transferase after infection during that period. Quantitative trait loci (QTL) were found for the different molecules studied. In population 41528, a stilbene synthesis QTL found on chromosome 18 co-segregated with Rpv2. Nevertheless, stilbene contents were higher in susceptible genotypes than in the resistant ones. In contrast, in population 41524, a stilbene synthesis QTL co-segregated on chromosome 12 with Rpv1 and their syntheses were positively correlated with the level of resistance.(5)