The influence of an intermittent lighting programme during the 12 h 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 mere randomly allocated to two open rooms At 5 days of age, an intermittent lighting schedule of 3 h of dark alternating with 1 h 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 1.5 ppm 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.