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Concentrations of growth hormone (GH), insulin, thyroxin(T4 ) and 3,3 5 -tri-iodothyronine (T3)in blood samples of growing yak during different seasons were determined by radio-immunoassay.Changes in bodyweight of the growing yak and compositions of grass were also measured. The seasonal changes in hormones were significant (at least P＜0.05). In the same season, variation in hormonal concentration was affected by the growth stage of the animal. The bodyweight gains varied in different seasons, with significant increase from May to September and decrease from January to May. Correlation analysis indicated that T4 had a significant positive correlation with bodyweight of the growing yak(r= 0.323,P＜0.05) ,but other hormones did not have significant correlations with bodyweight. The results show that annual cycle of weight loss and gain is attributable to the seasonal changes in feed supply and that the seasonal changes in the concentrations of the assayed hormones were indirectly dependent on grass growth.