The influence of an intermittent lighting programme during the 12h of the dark period on the incidence of ascites and on metabolic parameters of broiler chickens was examined. A group of 336 1-day-old male broiler chickens were randomly allocated to two open rooms At 5 days of age, an intermittent lighting schedule of 3h of dark alternating with 1h of light was introduced during the night in one room, while in the second room the schedule consisted of 23 h of light alternating with 1 h of dark. One-half of the birds in each room were fed a diet supplemented with 1.5ppm 3, 3', 5-triiodothyronine (T(3)), while the other half received a control diet. Mortality associated with ascites was significantly higher in birds fed the T 3 -supplemented diet, but was significantly lower in birds in the room receiving the intermittent lighting schedule. The intermittent lighting programme was also associated with a temporary reduction in growth rate, packed cell volume and plasma T(3) concentrations, suggesting reduced oxygen utilization. However, final body weights were not significantly affected by the lighting programme. It was concluded that imposing 12 h of intermittent lighting during scotoperiod reduces the incidence of ascite mortality.