Abstract: This study investigated effects of varying levels of light intensities (25, 10, 5, 2.5 and 0.2 lx) from 22 to 56 d of age at 50% RH on blood acid-base balance, metabolites and electrolytes of heavy broilers reared under environmentally controlled conditions. Four identical trials were conducted with two replications per trial. In each trial, 600 1-d-old Ross 308 chicks were randomly distributed into 10 environmentally controlled chambers (30 males and 30 females chicks/chamber). Each chamber was randomly assigned one of five light intensities from d 22 to 56 d of age. Birds were provided a four phase-feeding program (starter: 1 to 14 d, grower: 15 to 28 d, finisher: 29 to 42 d and withdrawal: 43 to 56 d). Feed and water were provided ad libitum. Venous blood samples were collected on d 21 (base line), 28, 42 and 56. The lowest light intensity of 0.2 lx significantly (p<0.05) increased pH, Na+, K+, Cl- and reduced pCO2, Hb and Hct. However, all these acid-base changes are still within the normal acid-base homeostasis physiological ranges. In addition, exposure of modern heavy broilers to varying light intensity produced no significant effect on pO2, sO2, Ca2+, mOsm, McHc, Angap, T3, T4 and CS. Acid-base regulation during light intensity exposure did not deteriorate despite a lower pCO2 which consequently increased blood pH that resulted in a compensatory mechanism for mild alkalosis. This study shows the positive impact on profits to commercial poultry facilities that are using low lighting environment to reduce hyperactivity, pecking damage and energy costs without the induction of physiological stress effects on broiler welfare.