Ten mouflon rams were kept with their mothers in a captive regimen under natural photoperiod. Seven of the mouflon lambs received a series of subcutaneous implants containing 18 mg of melatonin (Melovine ®). The first implant was inserted at the time of birth and it was followed by implant insertion every two months to maintain plasma melatonin concentrations above basal levels. Three mouflon lambs were used as controls. Body weight, outer horn length and horn base circumference were measured weekly. Three implanted lambs maintained a poor body condition throughout the experiment and died early. In the remaining four implanted lambs, the pattern of body and horn growth was similar to controls. Horn length and base circumference were correlated (P < 0.001) with body weight in both control and melatonin-implanted mouflons. No differences between control and survivor melatonin implanted mouflons were observed in weekly body weight gain (0.86 ± 0.19 kg and 0.77 ± 0.06 kg, respectively), weekly horn length growth (0.77 ± 0.03 cm and 0.77 ± 0.02 cm, respectively) and weekly base circumference growth (0.49 ± 0.03 cm and 0.45 ± 0.03 cm, respectively).