The primary objective of this study was to determine the duration of exposure to a long-day or short-day photoperiod required to disrupt photorefractoriness to short-day and long-day photoperiods, respectively. In Experiment 1, 4 groups of Suffolk breed ewes--designated B1, B2, B3, and B4--were placed in photochambers one day before the winter solstice, exposed to a 16L:8D photoperiod for 0, 30, 60, or 90 days, and then exposed to a 10L:14D photoperiod until the time of the summer solstice. Blood samples taken by venipuncture thrice weekly were analyzed for progesterone concentrations. The interval between start of the study and cessation of estrous cycles did not differ significantly between groups (p greater than 0.05). All 6 ewes in Group B1 then remained in anestrus for the duration of the study. Four of the 6 ewes in Group B2, and all ewes in Groups B3 and B4 resumed cycles after exposure to the 10L:14D photoperiod. In Experiment 2, 4 groups of ewes--designated A1, A2, A3, and A4--were placed in photochambers one day before the summer solstice, exposed to a 10L:14D photoperiod for 0, 30, 60, and 90 days, respectively, and then exposed to a 16L:8D photoperiod. Ewes in Group A1 started estrous cycles at a time not significantly different from ewes kept outdoors. However, onset of cycles was significantly advanced (p less than 0.05) in ewes exposed to 10L:14D. After ewes were returned to the 16L:8D photoperiod, estrous cycles were suppressed in 5 of 6 ewes in Group A2 and in all ewes in Groups A3 and A4.(ABSTRACT TRUNCATED AT 250 WORDS)