Effects of additive and dominant genes and their interactions on the inheritance of hectoliter weight in 10 sunflower hybrids developed by crossing five inbred lines derived from the synthetic NS-S-1 were analyzed in 2001 and 2002. The linkage among the expected progeny means was tested using the scaling tests method (Mather, 1949), while the estimates of gene effects and mode of inheritance were made by generation mean analysis (Mather and Jinks, 1982). The additive-dominant model was not adequate for all crosses from the two years. In the hybrids for which the model was not adequate, epistatic gene effects were important in the inheritance of the studied characters. In the first year of study, duplicate epistasis between dominant increasers was expressed in crosses C2 and C9, while duplicate epistasis between dominant decreasers occurred in crosses C5 and C6. In the second year of study duplicate epistasis between dominant increasers was expressed only in the cross C5, while duplicate epistasis between dominant decreasers occurred in crosses C1, C4, C6, C7 and C10. In the crosses C1, C3, C4, C7, C8 and C10 in the first year and in the crosses C2, C3, C8 and C9 in the second year of investigation the type of epistasis could not be determined, because the values of the non-fixable components (dominant and dominant × dominant) were insignificant.