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We have performed an "in vivo" study of storage lipid synthesis in developing sunflower seeds, from several high-oleic genetic backgrounds, using radioactive acetate in conjunction with methyl viologen as an inhibitor of the stearoyl-ACP desaturase. As such, some backgrounds showed stronger acyl-ACP thioesterase activity on stearoyl-ACP. We have developed a saturation coefficient that quantifies stearoyl-ACP thioesterase activity among sunflower lines based on their ability to synthesize saturated fatty acids under conditions when the competing stearoyl-ACP desaturase is inhibited by methyl viologen. The saturation coefficient is defined as the ratio of sum of the stearic, araquidic, and behenic saturated fatty acid contents to the unsaturated fatty acid content. On the basis of this coefficient, we were able to select high-oleic lines that, when crossed with the high-stearic CAS-3 line, developed progeny with high-stearic content on a high-oleic background. This approach has enabled us to identify lines with a combination of alleles that synthesized oils with more stearic acid in a high-oleic background, 21% stearic and 62% oleic contents. In contrast, lines with a lower index produced progeny that contained less stearic acid, similar to those obtained previously, that were 13% stearic acid content in high-oleic background. This method could also be used for other metabolic pathways where the blockage of a principal pathway may activate a secondary pathway. However, it should be emphasized that although the stearic acid content could be augmented it was not possible to break the association or the epistatic relationship that exists between the genes that permit a high-stearic phenotype and those that determine a high-oleic background.