Abstract. The fatty acid profile and the related indices of the nutritional quality of breast and thigh muscles were studied in two lines of chickens – La Belle (LB) and White Plymouth Rock (WPR) – slaughtered at the age of 9 and 18 weeks. The fatty acid profile was more affected by the age than the line of the birds; however, the influence of both differed between the breast and thigh. The content of total saturated fatty acids (SFAs) increased in the thigh (P <  0.01), while that of the monounsaturated fatty acids (MUFAs) decreased with age in both muscles (P <  0.001). This corresponded to the significant decrease in C18:1 in the older chickens and the lower desaturase activity (P <  0.001). The contents of C20:4n-6, C22:5n-3 and C22:6n-3 and the total amount of polyunsaturated fatty acids (PUFAs) in breast were higher (P < 0.001) at the age of 18 weeks. A similar pattern in the individual and total PUFA was observed in the thighs. The effect of line was more visible in the breast, leading to a lower C14:0 content and C20:5n-3 and a higher C18:0 content in the WPR chickens (P <  0.001), corresponding to the higher elongase and thioesterase indices in these birds. Both atherogenic (AI) and thrombogenic (TI) indices were lowered, while the ratio of hypocholesterolemic ∕ hypercholesterolemic fatty acids (h ∕ H) and polyunsaturated ∕ saturated fatty acids (P ∕ S) increased in the breast of the birds at 18 weeks. In breast and thigh meat, the ratio of n-6 ∕ n-3 PUFA decreased in the older chickens (P <  0.001).