Long photoperiods during established lactation increase milk production in dairy cattle and dairy sheep, but recent research in cattle and dairy goats suggests an additional influence of prepartum day length on milk yield in the subsequent lactation. The proposed mechanism of function is the level and role of circulating prolactin in mammary development. The objectives of this study were to evaluate the effect of prepartum photoperiod on milk production, milk composition, and prolactin concentration of 22 multiparous dairy ewes exposed to short day prepartum photoperiod (SDPP; 8 h of light:16 h of dark) or long day prepartum photoperiod (LDPP; 16 h of light:8 h of dark) for at least 6 wk prepartum. During the first 8 wk of lactation, SDPP ewes tended to produce more milk than LDPP ewes (2.43 vs. 2.29 kg/d, respectively), and the milk of SDPP ewes had a greater fat percentage than that of LDPP ewes (6.04 vs. 5.51%, respectively). Due to daily milk yield and greater fat content, SDPP ewes produced more 6.5% fat-corrected milk (+0.30 +/- 0.08 kg/d) and 6.5% fat- and 5.8% protein-corrected milk (+0.28 +/- 0.08 kg/d) than LDPP ewes. For the lactation period of 180 d, SDPP ewes produced more test day milk than LDPP ewes (1.76 vs. 1.60 +/- 0.05 kg/d, respectively), but there were no differences in milk fat or protein percentages. Ewes in both treatments experienced a prolactin surge at lambing, but SDPP ewes had lower circulating prolactin concentration than LDPP ewes from 4 to 0.5 wk before lambing (14.7 vs. 51.3 +/- 4.2 mg/dL, respectively). These data suggest that decreased prepartum photoperiod may be important for increasing milk production in dairy ewes and may provide a management strategy for dairy sheep producers to increase milk yield.