Three experiments were conducted to examine the impact of changing photoperiod length on the performance of chicken broilers. Effect of long, constant (23-h) photoperiod length (23H) was compared with treatments where photoperiod increased from 6 to 23 h abruptly after 21 days (6H) or gradually during the experiment (INC). The 6H program was used in all three experiments, whereas the INC treatment was included only in Experiment 3. Differences between total body weight gains of birds in the 6H and 23H treatments were inconsistent and small in the three experiments. In Experiment 3, there were no differences between total body weight gains for INC, 6H, and 23H treatments. Feed-to-gain ratios for 6H broilers were significantly superior to those for birds on the 23H treatment for Days 0 to 21 (all experiments), Days 21 to 42 and Days 0 to 42 (Experiment 2), and inferior for Days 21 to 42 (Experiment 3). The INC treatment was significantly superior to the 23H program for all time periods and to 6H birds for Days 21 to 42 and Days 0 to 42. The 6H treatment significantly reduced the incidence of leg abnormalities in comparison with the 23H program in all experiments; the INC and 6H treatments were equal in incidence in Experiment 3. When data from the three experiments were combined, mortality due to sudden death syndrome and other causes for the 6H treatment was significantly lower than that of the 23H program; values for the 6H and INC treatments were not different in Experiment 3.(ABSTRACT TRUNCATED AT 250 WORDS)