Experiments were conducted to determine whether dietary 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] will alleviate a cholecalciferol deficiency induced by low dietary cholecalciferol and no fluorescent lighting and to determine cholecalciferol requirements as influenced by fluorescent lighting or 1,25-(OH)2D3. In each study, nutritionally complete basal diets were fed to broiler cockerels from 1 to 16 d of age. Experiment 1 had a 2 x 2 x 2 factorial arrangement of treatments with 1,25-(OH)2D3 at 0 and 10 micrograms/kg, cholecalciferol at 2.75 and 27.5 micrograms/kg, and fluorescent lights on or off. Experiments 2 to 4 had four levels of dietary cholecalciferol (0, 5.0, 27.5, and 50.0 micrograms/kg) and fluorescent lights on or off (Experiment 2) or 1,25-(OH)2D3 at 0 and 10 micrograms/kg (Experiments 3 and 4). In Experiment 1, fluorescent lighting increased bone ash, and decreased the incidence and severity of rickets at 2.75 micrograms/kg cholecalciferol and 0 microgram/kg 1,25-(OH)2D3 and reduced the severity of TD at both levels of cholecalciferol and 0 microgram/kg 1,25-(OH)2D3. In all cases 1,25-(OH)2D3 improved bone ash. The metabolite also decreased the incidence and severity of TD at both cholecalciferol levels with lights off and decreased the incidence and severity of rickets at 2.75 micrograms/kg cholecalciferol and lights off. In the absence of fluorescent lighting and 1,25-(OH)2D3 27.5 micrograms/kg cholecalciferol reduced the incidence and severity of rickets to levels equivalent to those produced by either fluorescent lighting or 1,25-(OH)2D3 alone (Experiments 2, 3, and 4). However, even 50.0 micrograms/kg cholecalciferol was not as effective as fluorescent lights or 1,25-(OH)2D3 in reducing the incidence and severity of TD.