The 13C turnover rates of the liver and thoracic pectoral muscle of growing broilers were determined by feeding diets with varying 13C content. Male chicks (1-d-old) were subjected to treatments based on free choice of 5 different mixes of energy and protein sources from plants with C3 and C4 photosynthetic pathways that had differing 13C content. Rice bran (R) and soybean meal (S) were the C3 sources, while maize (C) and maize gluten meal (G) were the C4 sources. Choices were R + S, C + G, R + G, C + S or R + C + G + S. The 6th treatment was a complete feed (CF) that was similar to a commercial broiler feed. The isotopic composition of the birds' tissues was representative of the isotopic composition of the diets. The assimilation was faster for C3, in both liver and muscle, than for C4 diets, and give the delta per mil difference between the diet and tissues. The liver is the most active metabolic tissue and gave more rapid isotope turnover than in muscle.