The fatty acid composition of the plasma free fatty acid and adipose tissue triacylglycerol fractions was determined in lean and fat selection lines of Texel-Oxford and Scottish Blackface sheep at the 6th year of divergent selection. The mean proportion of 18[ratio ]2n-6 in the triacylglycerol of subcutaneous backfat was 1·3-fold higher in the phenotypically fatter sheep in the fat lines than in the phenotypically leaner sheep in the lean lines. Regression analysis indicated a positive relationship between this fatty acid and backfat depth whereas the proportion of 18[ratio ]1n-9 in the tissue triacylglycerol was negatively related to fatness. The proportions of 18[ratio ]2n-6 and of other polyunsaturated fatty acids in the plasma free fatty acid fraction were much higher than in adipose triacylglycerol. For the Scottish Blackface sheep in the fed state, the mean proportion of 18[ratio ]2n-6 in plasma free fatty acid (measured on all sheep) was 1·4-fold greater in the fat line than in the lean line. Regression analysis indicated a positive relationship between the plasma content of this fatty acid and backfat thickness whereas the proportion of 18[ratio ]1n-9 in plasma free fatty acid showed a negative relationship with fatness. The relationship between the plasma proportion of 18[ratio ]2n-6 and fatness was not observed after 48 h of fasting; instead, the plasma proportion of 18[ratio ]0 was found to be positively related to fatness in the fasted state. In summary, this paper shows how plasma and adipose tissue fatty acid profiles differ, and it quantifies the effects of selection on the plasma profiles. Possible reasons for the difference in fatty acid profiles between adipose tissue and plasma are discussed in the paper. It is suggested that plasma 18[ratio ]2n-6 levels during the early post-weaning growth period should be investigated as indicators of future fatness.