This research communications addresses the hypothesis that a part of iso 17:0 and anteiso 17:0 in milk fat could come from endogenous extraruminal tissue synthesis. In order to confirm this a linear regression model was applied to calculate the proportions of iso 17:0 and anteiso 17:0 in milk fat that could come from elongation of their putative precursors iso 15:0 and anteiso 15:0, respectively. Sixteen dairy goats were allocated to two simultaneous experiments, in a crossover design with four animals per treatment and two experimental periods of 25 d. In both experiments, alfalfa hay was the sole forage and the forage to concentrate ratio (33 : 67) remained constant. Experimental diets differed on the concentrate composition, either rich in starch or neutral detergent fibre, and they were administered alone or in combination with 30 g/d of linseed oil. Iso 15:0, anteiso 15:0, iso 17:0 and anteiso 17:0, the most abundant branched-chain fatty acids in milk fat, were determined by gas chromatography using two different capillary columns. The regression model resolved that 49% of iso 17:0 and 60% of anteiso 17:0 in milk fat was formed extraruminally from iso 15:0 and anteiso 15:0 elongation.