In this work, we investigated the effect of epiphysectomy in inhibiting and activating the melatonin-forming function on hemocoagulation factors. When the melatonin-producing function of the pineal gland is activated in the dark phase, the blood clotting time is shortened more sharply than in animals in a state of inhibition of the melatonin-producing function of the pineal gland. The reasons for this phenomenon are revealed in the following: when the melatonin-forming function of the pineal gland is inhibited in the light phase, a hypercoagulable shift is observed in almost all hemocoagulation factors - blood clotting time is shortened (by 38%), the number of platelets increases (by 3.1%), the amount of fibrinogen increases (by 25%), fibrinolytic activity decreases (by 32%). In the dark phase, hypocoagulative changes develop, the number of platelets decreases by 2.7%, the amount of fibrinogen increases by 45%, fibrinolytic activity increases by 60%. The effect of light on the endocrine apparatus is mediated by two special organs: photoreceptors and neuroendocrine transducers. Moreover, the only photoreceptor in adult mammals is the retina. When light acts on the retina, nerve impulses enter not only the brain, but also through the spinal cord into the neuroendocrine transducers-epiphysis. Under the influence of light in rats, the size of pinealocytes decreases, the number of nucleoli in cells decreases, and the cytoplasm becomes less basophilic. The above makes it possible to conclude that with prolonged inhibition of the pineal gland by the light factor, hypercoagulation develops. With prolonged activation of the melatonin-producing function of the pineal gland, a paradoxical phenomenon is observed: the blood clotting time is sharply shortened (by 62%) than in animals with an inhibited melatonin-producing function of the pineal gland and epiphysectomized animals.