An experiment with 840 day-old male broiler chicks (Ross) was initiated to investigate the effect of intermittent Lighting schedules, combined with two isocaloric feeds differing in protein content, on ascites mortality. At 9 d of age, chicks were randomly distributed over two rooms: one room with a 23 h light(L):1 h dark (D) lighting schedule (CL), and another room with an intermittent lighting schedule (IL, 1L:3D). In each room, two isocaloric feeds differing in CP content, supplemented or not supplemented with 1.5 ppm triiodothyronine (T-3), were provided. The experiment was repeated under identical conditions except that lighting schedules were changed between rooms. From Day 14 onwards, biweekly growth and feed intake were determined and feed conversion (FCR) was calculated. Daily mortality was recorded and necropsies were performed. Weekly blood samples were taken from 10 randomly chosen birds per experimental group for measurements of growth hormone (GH), thyroid hormones [thyroxine (T-4) and triiodothyronine (T-3)], and hematocrit values.