A study on male turkeys was conducted to evaluate the physiological and growth effects of organic acids, organic acids with essential oils or herbal extract added to diets. The control diet (C) contained premix without any feed additives. Three experimental diets were fed from 1 to 140 days of age: diet A with a blend of organic acids (formic acid and propionic acid) in the amount of 5.0 g/kg of feed, diet AO with a blend of organic acids (citric, fumaric, orthophosphoric and malic acid) and with a mixture of essential oils (from citrus fruits, cinnamon, oregano and thyme) in the amount of 0.5 to 1.0 g/kg of feed, diet HE with preparation composed of oregano essential oil, spice extracts of turmeric and capsicum in the amount 0.3 g/kg of feed. Gastrointestinal tract development and metabolism (after 56 days of feeding) as well as growth performance and carcass characteristics (after 140 days of the experiment) were evaluated. Dietary supplementation with A, AO or HE significantly decreased the pH of the crop contents, but had no effect on the pH of the caecal digesta. Dietary supplementation with A or HE increased the activity of α-glucosidase in the ileal digesta. The tested supplements had no influence on SCFA production in the caeca. All supplements increased the body weights of turkeys, but the differences were significant (P≤0.01) only at the age of 84 days. The final body weight (at 140 days of age) of turkeys fed diets with the tested supplements were higher by 3.3-3.9%, and feed conversion ratio decreased by 2.4-5.3% (statistically non-significantly) in these treatments, compared to the control. Generally, carcass traits were not influenced by the above feed supplements.