Abstract. Previous studies demonstrated that milk yields in sheep displaying strong seasonal sexual activity depend on the day length. The objective of the studies was to determine whether the introduction of melatonin in high pregnancy affects milk secretion in seasonally sheep. The studies were carried out on 60 Polish Longwool sheep. Sheep were allocated to three groups: Group I (n = 20 – the control group, lambed in February), Group II (n = 20 – a group of sheep lambed in June and kept under natural day-length conditions), Group III (n = 20 – a group of sheep with melatonin implants injected six weeks before lambing, sheep lambed in June). Lambs were reared with mothers up to 56th day of their life. When lambs were weaned, ewes were milked mechanically twice a day up to the dry period. Once a month collective milk samples were drawn from six sheep from each group in order to determine the concentration of melatonin. Milk yields were subjected to individual checks every 10 days. The studies demonstrated that sheep lambed in February (Group I) displayed the highest milk yields in the milking period (37.8 ± 8.1 l). The milk performance of the two other groups was lower and amounted to 30.2 ± 9.4 litres in case of sheep lambed in June and to 29.2 ± 7.6 litres in sheep with melatonin implants. The introduction of melatonin signal to produce a short-day condition in state of high pregnancy in ewes caused a drop of milk yields both in the period of lambs raising and during milking.