2. Two diets were offered to broiler chickens from d old to 21 d of age. The control diet (C) was slightly lower in metabolisable energy (12.13 MJ/kg ME) and crude protein (215 g/kg CP) than breeders’ recommendation. The second diet, made as XT 6930, was added on the top of the control diet at 100 mg/kg. Each diet was offered ad libitum to birds housed in one of 10 floor pens in a randomised complete block design. The birds were housed in 20 floor pens, 10 birds in each pen, and were allocated to 10 replicates of the two dietary treatments.

3. The concentration of antioxidants in the liver of the birds was determined using high-performance liquid chromatography (HPLC) at 21 d of age. Birds fed control diet only had lower weight and converted less efficiently feed to gain compared to birds fed essential oils-supplemented diet. Feed consumption was not affected by dietary treatments. The antioxidant data showed that supplemented essential oils improved the hepatic concentration of carotenoids and coenzyme Q₁₀ when fed to broiler chickens. The morphometry of the caecal tonsils of the birds was not influenced by dietary treatments.

4. It can be concluded that that dietary combination of essential oils, including carvacrol, cinnamaldehyde and capsicum oleoresin, improved growth, feed efficiency and the hepatic concentration of carotenoids and coenzyme Q₁₀ when fed to broiler chickens.