To explore the correlations between the seasonal changes of gastrointestinal hormones and melatonin in the pineal gland，and reveal the connotation of spleen corresponding to late summer.Methods:SD male rats were raised in batches in spring，summer，late summer，autumn，and winter.The rats in each season were randomized into a control group(without any treatment) and a model group(with removal of the pineal gland).The samples were collected on the Spring Equinox，Summer Solstice，one day before the Beginning of Autumn，Autumn Equinox，and Winter Solstice，and then the serum levels of melatonin(MT)，gastrin(GAS)，and motilin(MTL) were measured by enzyme-linked immunosorbent assay.Results:The serum levels of MT，GAS，and MTL in the control group presented seasonal changes，being the lowest in late summer among the five seasons.After removal of the pineal gland，the seasonal rhythm of MT，GAS，and MTL changed，while the lowest levels still in late summer.The secretion of GAS and MTL was consistent with that of melatonin during late summer.Conclusion:The digestive function of the gastrointestinal system was closely related to the TCM theory of visceral manifestations regarding spleen.The seasonal changes of gastrointestinal motility under the regulation of melatonin secreted by the pineal gland provide a basis for revealing the physiological mechanism of the TCM theory of spleen corresponding to late summer.