We investigated whether the genetic difference in plasma melatonin concentration in ewes was due to differences in the synthesis pathway from the pineal gland or in the catabolism of the hormone. Two groups of ewes [9 low (L) and 10 high (H)] were selected according to the breeding value of their mean nighttime plasma melatonin concentrations estimated at winter and summer solstices. In response to an identical dose of melatonin administered intravenously at 9:00 AM, no differences between groups were observed for any of the kinetic parameters: clearance rate, steady-state volume of distribution, terminal half-lives, and mean residence times. In the second experiment, two series of frequent blood samples were performed, one in the middle of the dark phase with samples taken every 5 min, and the other over 24 h with hourly samples. Highly significant differences between groups in nocturnal melatonin production rate were observed (L: 25.7 ± 2.8 vs. H: 63.1 ± 8.9 µg · kg¹ · h¹, P < 0.01). Thus the genetic differences in plasma melatonin concentrations in ewes originate in the synthesis pathway of the melatonin from the pineal gland rather than from differences in the catabolism of the hormone.