To determine the ovarian response to the ram effect after treatment with melatonin, on 8 March, 71 Rasa Aragonesa ewes were randomly assigned to either the treatment group and given an 18mg melatonin implant or the untreated group. On 19 April (day 0), rams were introduced into the flock. Melatonin treatment produced a significantly higher percentage of cyclic ewes at ram introduction (P<0.05). Melatonin-treated ewes had their first oestrus after ram introduction significantly earlier than did untreated ewes (P<0.0001), and the groups differed in the distribution of their ovarian response. Most (80%) of the treated ewes exhibited a silent ovulation followed by a cycle of normal duration, whereas about half (52%) of the untreated ewes exhibited a silent ovulation, a short cycle, and another silent ovulation followed by a cycle of normal duration (P<0.05). At ram introduction, melatonin-treated ewes, cyclic and non-cyclic, had higher mean plasma progesterone concentrations than did untreated ewes. The proportion of ewes that mated within the first 17 days of the mating period was significantly higher among the treated ewes than in the untreated ewes (P<0.0001). Furthermore, at lambing, 39% of the melatonin-treated ewes lambed within the first 17 days of the lambing period, while none of the untreated ewes lambed in that period. The untreated group exhibited peaks in mating between days 18 and 21, and particularly, between days 22 and 25, when the majority of ewes lambed; peaks did not occur in the treatment group. Treated and untreated ewes did not differ significantly in fertility, litter size and fecundity. In conclusion, melatonin treatment modifies the ovarian response to the ram effect in ewes, which leads to modifications in mating patterns, and consequently, the lambing curve.