Two genotypes of male broilers were given 12 h of daily illumination; as a conventional photoperiod, with the final hour at reduced illuminance to simulate dusk, or with 1 h of the light given during the middle of the night. The lighting modifications had no significant effect on any performance variable or on tibial breaking strength. Feed intake was unaffected by the lighting treatments during either the 1-h dusk period or the night, but was inexplicably stimulated in the both experimental groups during the main photoperiod.