Objective To investigate the correlation between different solar terms in immunoregulation and the melatonin（Mel）-pineal gland.Methods Forty days before vernal equinox

autumn equinox

summer solstice

and winter solstice respectively

96 Wistar rats were randomized into normal group

operation group

and sham operated group

with 8 in each.The operation group was made models with pineal gland extracted one month before the vernal equinox

autumn equinox

summer solstice

and winter solstice respectively.The sham opertaed group was given operation but the pineal gland was not removed.Three groups were fed under the same condition to the day of vernal equinox

autumn equinox

summer solstice

and winter solstice

and were killed at 20 o’clock for measuring the Mel content of serum and lung tissues in the four seasons.Results In the normal group

the Mel content in serum in spring was significantly higher than in summer

autumn and winter（P<0.01）.It was significantly lower in summer than in autumn（P<0.05） and in winter（P<0.01）.The Mel content in the lung tissues in spring was significantly higher than in the other three seasons（P<0.01）.It was significantly higher in winter than in summer（P<0.05）.The serum Mel content of the normal group in spring and summer was significantly higher than in the sham operated group（P<0.05）

and the Mel of the operation group was higher than that of the sham operated group in summer （P<0.05）.It was significantly higher in the sham operated group in spring than in the normal and operation groups（P<0.01）.Conclusion Under the normal condition

the Mel content in serum and lung tissues of rats is the highest in spring and lowest in summer.Being the important medium

the pineal gland joins the regulation mechanism of "lung corresponding to autumn" theory in TCM.