Length of melatonin exposure and onset of ovarian activity in anoestrous ewes.

Adult anoestrous ewes (N = 30) were given intravaginal Silastic implants containing melatonin or empty implants (N = 7) during mid-anoestrus (4 July). Implants were removed 16 days later (Group 1), 36 days later (Group 2) or 93 days later (Group 3). Blood samples were taken twice weekly for progesterone assay to monitor onset of ovarian activity. The percentage of ewes in each group showing early onset of ovarian cyclicity was significantly correlated with length of exposure to melatonin.