The seasonal changes in circulating melatonin were determined in ewes and rams exposed to the natural photoperiod at 52°N during first year and in rams under the artificial daylength conditions (16D:8L), during the second year of the experiment. The changes in the luteotropin (LH) and testosterone (T) concentrations in blood plasma of rams were also monitored. The mean daily concentrations of plasma melatonin in the ewes varied through a year from 41 ± 7 to 112 ± 13 pg/ml and a significant increase above the daily values occured during the first hour after sunset. The duration of nightly melatonin secretion was longer during the period of short day (10-12h), than under the long day conditions (8-10h). The mean nightly concentrations of circulating melatonin were highest from August to February: 171 ± 73 to 329 ± 187 pg/ml for individual ewes. The mean concentrations of LH and testosterone in blood plasma of rams, under the natural photoperiod, were low during the period of sexual quiescence and the significantly high levels occured in August: 6.2 ± 0.4 ng/ml and in October: 4.6 ± 0.7 ng/ml, respectively. Shortening of daylength from 16 to 8 h in May evoked in rams an increase in duration of melatonin secretion from 6 to 14 h. This advanced reproductive function by 6 to 9 weeks, with the peak of LH and testosterone secretion noted at the beginning of July: 8.2 ± 0.9 ng/ml and in August: 5.2 ± 0.9 ng/ml, respectively. Nevertheless the duration of reproductive activity of rams during the second year of the experiment was shorter than under the natural photoperiodic conditions.