The aim of the present study was to determine the effect of prolonged photoperiod during late pregnancy on subsequent ovarian activity and fertility in mares. Pregnant mares (n=13) due to give birth in January and February were stimulated by a fixed photoperiod (16 h light: 8 h dark) from 15 November (during the last 2-3 months of gestation) until up to 1 month after parturition. A control group of mares (n=9) due to give birth at the same time were kept in the same stable and management regimen, but under natural light conditions. Light-treated mares ovulated during foal oestrus approximately 3 days earlier than did the control mares (P < 0.05). The incidence of winter ovarian inactivity in foaling mares was 7.6% in the treatment group and 33% in the control mares. The processes of involution and uterine fluid expulsion were similar in both groups. The final pregnancy rate was high in both the experimental and control groups, being 92 and 89%, respectively. However, the duration of the open period (period from parturition to next pregnancy) was significantly shorter in the mares that received the light treatment compared with the untreated control mares.