Abstract: The differences were studied among seasons, reproductive rhythms, breeding active and resting terms. The plasma concentrations of melatonin of adult female yaks were determined by reversed-phase high-performance liquid chromatography. A total of 63 adult female yaks were selected as experimental animals and divided into three groups: A with calving every year and breeding active term, B with calving per two years and full milking and breeding resting term, C with calving per two years and semimilking and breeding active term. Each group had 21 yaks. In the sampling period of July 21-23 in warm season and November 21-23 in cold season, the yaks were exposed to 12.00 L∶12.00 D and 14.65 L∶9.35 D, respectively. Blood samples were collected for 50 minutes starting at 2 h before sunset. A quantity of 20 mL jugular vein blood was drawn from every animal and anticoagulated with heparin sodium. Plasma obtained after centrifugation was estimated for melatonin. The content of plasma melatonin of group A ((68.90±5.82) pg/mL) was higher than that of group B ((44.38±2.61) pg/mL) (P<0.01) and group C ((57.51±3.82) pg/mL) (P<0.05). The content of plasma melatonin in cold season was lower than that in warm season (P<0.05). The level of plasma melatonin of yak cows in reproductive active term was higher than that of it in fallow term (P<0.05). There was a seasonal rhythm in the secretion of plasma melatonin of yak cows. The concentration of plasma melatonin had an effect on the seasonal breeding and reproductive rhythms of female yaks.