Production values (PVs), defined as the weight of the end product/weight of the substrate required for carbon skeletons and energy production, were calculated for plant fatty acids. The PVs varied from 0.361 to 0.300 with linolenic acid having the lowest value. In general, the PVs of unsaturated fatty acids were lower than those of saturated fatty acids of similar chain lengths. Using this basic information, PVs of (A) oils from different oilseed crops, based on their standard fatty acid composition and (B) seed biomass with specified oil content and fatty acid composition were calculated. 1/PV gives the glucose required for the biosynthesis of 1 g end product and thus an estimate of the photosynthate requirement for the desired breeding goal can be estimated. Such calculations show that increasing oil percentage in seeds has a maximum energy cost when the increase in oil is associated with a decrease in the amount of carbohydrates where there is no change in protein concentration. Reduction of erucic acid content in the rapeseed oil did not alter its PV. It is inferred that there are no serious bioenergetic constraints in altering the fatty acid composition.