Between June 1 and August 24 of two successive summers, Targhee ewes (n = 64) and Finn x Targhee ewes (n = 44) were subjected to different photoperiods. Treatments were natural day length (ND), eight hours light: 16 hours of dark (8L:16D), 16L:8D shifted to 8L:16D (16L:8D-8L:16D) and seven hours of light:nine hours of dark:one hour of light:seven hours dark (7L:9D:1L:7D). Days to the mean first breeding mark were shortest (P<0.05) for the 16L:8D-8L:16D group (32 days) followed by the 7L:9D:1L:7D group (39 days). The longest mean intervals to marking were 52 (ND) and 47 days (8L:16D). Time to mean first breeding mark was 18 days shorter (P<0.05) in 1981 than in 1982. Mean serum progesterone values were </=1 ng/ml and did not differ (P>0.05) among treatments through week 6. The 8L:16D group had the most rapid rise and the highest terminal progesterone value, while the ND group had the slowest rise and lowest terminal value. Mean serum prolactin values in general started high and decreased over time. This decrease in all treatments preceded the rapid rise in progesterone. The most rapid prolactin decline was for the 8L:16D group. The percentage of exposed ewes lambing was lowest (P<0.05) for ND ewes (37%) and highest for 8L:16D ewes (81%). The shortest mean interval from the start of treatment to lambing was 210 days (7L:9D:1L:7D) followed by 214 days (8L:16D), 217 days (ND) and 218 days (16L:8D-8L:16D). Considering percentage lambing in combination with interval to lambing, the 8L:16D treatment was the most effective treatment.