Abstract: In order to make clear the law of annual cycle of the function of thyroid in cobra,the writer of this article chose 84 mature healthy male Naja naja Atra bought from Suichang,Zhejiang Province,divided them into 12 groups and drew their blood during the period of 9:00-11:00 a.M.On around the 20th day of every month to be studying materials.When the blood drawn had been made into plasma,it was stored in a low-temperature refrigerator -35 ℃.As the specimens of every month were all at hand,the T3,T4,TsH concentration in plasma during the whole year were tested by way of radioimmunoassay.The instrument used was an automatic γ-immune counter of model FJ-2008G,made in the Statewned 262 Factory.The T3,T4,reagent box was made in Shanghai Living Thing By-product Works under Public Health Ministry.The TsH reagent box was made in Shanghai Chemical Reagent Institute.Every determination was made according to the directions contained in the determination boxes.The relationship between cobras activity grades and concentration of T3,T4,was counted by way of χ2 tendency test.The correlation coefficient between the concentration of T3 and T4 was counted by way of linear correlation.The laws of the annual cycle of concentration of T3,T4 and TsH were counted by way of circular distribution statistics.The results came out as follows:(1)The concentration of T3,T4,TsH in cobra plasma all presented annual cycle of single-peak curve.The peak figure of plasma TsH concentration fell on April 16,The 95% reliability section of peakfigure of whole cobra appeared during the period of March 5 to May 27.The peak of plasma T3,T4 concentration appeared on July 2 and July 8 respectively.The 95% reliability section of peak-figure of whole cobra fell on May 13 to August 21 and June 29 to July 17 respectively.The peak figure of plasma TsH concentration of cobra appeared more than 2 months early than that of the plasma T3,T4 concentration.This indicated that cobra plasma TsH has promoting function to the systhesis and secretion T3,T4.(2)The internal cause of the increase of T3,T4 secretion is the rhytmic self-adjustion of the cobra.TsH in the cobra is the promoter of the structural change of the thyroid.(3)The plasma T3,T4 concentration in the cobra has very significant relationship with the ambient temperature and the activity grade of cobra.