This study was conducted to examine the effect of supplementation of 0 (control-), 0.5, 1, 1.5, 2, and 2.5 g · l⁻¹ essential oils of Satureja khuzistanica (EOSk) or 3 g · l⁻¹ polysorbate-80 (control+) into drinking water using 420 one-day-old Cobb 500 broiler chicks up to day 28 of age. The EOSktreated water significantly suppressed the productive performance of the birds (P < 0.05). Addition of EOSk to water at 2.5 g · l⁻¹ caused significant increases in liver weight and serum glutamate pyruvate transaminase activity (P < 0.05). A significant decrease was found in the average serum uric acid concentration for the birds receiving water supplemented with EOSk at 0.5 to 2.5 g · l⁻¹. It is concluded that persistent inclusion of EOSk at 0.5 to 2.5 g · l⁻¹. into drinking water suppressed performance of broiler chickens during the early growth periods. No indication was found of hepato-suppressive effects of EOSk up to g · l⁻¹.