To investigate both seasonal changes and possible intracorporal gradients of phospholipid fatty acid composition, skeletal muscles ( n=124), hearts ( n=27), and livers ( n=34) from free-living brown hares (Lepus europaeus) were analyzed. Phospholipids from both skeletal muscles and heart had a high degree of unsaturation with 66.8+/-0.63% and 65.7+/-0.5% polyunsaturated fatty acids, respectively. This is the highest proportion of polyunsaturated fatty acids reported in any mammalian tissue. Polyunsaturated fatty acid content in skeletal muscles was 2.3% greater in winter compared to summer (F(1,106)=17.7; P=0.0001), which may reflect thermoregulatory adjustments. Arachidonate (C20:4n-6) showed the greatest seasonal increase (+2.5%; F=7.95; P=0.0057). However, there were no pronounced differences in polyunsaturated fatty acid content between skeletal muscles from different locations in the body (m. iliopsoas, m. longissimus dorsi and m. vastus). Total muscle phospholipid polyunsaturated fatty acid content was correlated with polyunsaturated fatty acid content in triacyglycerols from perirenal white adipose tissue depots ( r(2)=0.61; P=0.004). Polyunsaturated fatty acids were enriched in muscle phospholipids (56.8-73.6%), compared to white adipose tissue lipids (20.9-61.2%), and liver phospholipids (25.1-54.2%). We suggest that the high degree of muscle membrane unsaturation is related to hare-specific traits, such as a high maximum running speed.