The aim of this study was to describe plasma melatonin (MLT) profiles in mares from spring equinox to summer solstice. Two experiments were performed on 19 warm-blood mares: the first in a veterinary clinic and the second in a stable without electricity and located outside a village. In the first experiment, blood samples were collected at 2-h intervals during 50-h periods in March, April, May, June, and July. In the second experiment blood samples were drawn at 2-h intervals for 26 h in March. Plasma concentrations of MLT were measured by direct RIA. The mean plasma MLT level showed significant (P≤0.05) time-dependent changes in horses sampled in March, April, May, June, and July. In experiment I, the mean level of MLT was significantly elevated (compared to the day-time values at 12.00, 16.00, and 18.00) between 20.00 and 04.00 in March, and between 22.00 and 04.00 in April and May, between 24.00 and 04.00 in June and July. Therefore it could be concluded that the duration of the elevated MLT level reflects the length of scotophase, and merits consideration as an important signal in seasonal regulation of reproductive activity. Individual profiles of MLT level were well-entrained in the majority of animals studied in experiment I and in all individuals used in experiment II. The concentrations of plasma MLT varied between mares; however, in the vast majority of individuals, the plasma concentration of MLT was low compared to other mammalian species.
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