An experiment was conducted to investigate the effect of relative humidity (RH) at chronic temperature on growth performance, glucose consumption, and mitochondrial adenosine triphosphate (ATP) production of broilers.  A total of 180 28-day-old Arbor Acres broilers (half males and half females) were randomly allocated to three treatments, each containing six replicates of 10 birds per treatment, using a completely randomized design.  Birds were reared at 35, 60 or 85% RH at 32°C for 15 days (temperature increased by 3°C every 3 days from 20 to 32°C within 15 days: 20–23–26–29–32°C).  RH affected (P<0.05) average daily feed intake (ADFI), average daily gain (ADG), average daily water consumption (ADWC), blood glucose concentrations, muscle glycogen levels, avian uncoupling protein (av UCP) mRNA expression, and cytochrome c oxidase (CCO) activity in liver of broilers at 42 days of age.  The 85% RH decreased (P<0.05) ADFI, ADG and ADWC; 35% RH decreased (P<0.02) ADG.  Both 85 and 35% RH increased (P<0.01) blood glucose and decreased (P<0.05) muscle glycogen.  Both 85 and 35% RH increased (P<0.05) av UCP mRNA expression.  35% RH decreased (P<0.05) CCO activity.  In conclusion, both high and low RH inceased glucose consumption and reduced mitochondrial ATP poduction, leading to a decline in growth rate.