The myocardial phospholipid fatty acid (FA) composition (mol %) of 7 avian species was determined, in a body mass range from 150 g (Japanese quail, Coturnix coturnix japonica) to 19 kg (turkey, Meleagris gallopavo). Significant allometric increases were found for C16:1 n7 (allometric exponent: B=0.15), C18:1 n7 (B=0.08), C18:1 n9 (B=0.24), C20:1 n9 (B=0.22) and C20:3 n3 (B=0.12); moreover, total monounsaturates (B=0.20) and the sum of n9 FAs (B=0.24) was also positively related to body mass. The total n3 FAs (B=-0.36), and within them C22:5 n3 (B=-0.41) and C22:6 n3 (B=-0.60) showed allometric declines, such as total polyunsaturated fatty acids (PUFA; B=-0.01), unsaturation index (B=-0.03) and mean FA chain length (B=-0.003). Comparing our results with earlier published data on avian skeletal muscle and divergent mammalian tissues, the allometric scaling of the above membrane forming fatty acids seems to be part of a general relationship postulated as the theory "membranes as metabolic pacemakers". In addition, the cardiac muscle malondialdehyde concentration was negatively related to body mass (B=-0.16), referring to a lower level of lipid peroxidation in larger birds, and vice versa, indicating a progressive myocardial lipid peroxidation in smaller-bodied species.