Considering the physiological role of epiphysis in functional activity and defense reactions of the body, this study was carried out to evaluate changes of lithium level due to biorhythms of epiphysis. For this purpose, albino mice (6-month old) were used. The animals were kept in light (resulting in inhibition of the activity of epiphysis) and dark (which increases the activity of epiphysis) environments (March, April, and May months). After a week, the dynamics of the changes of the lithium level in their blood and in plasma were evaluated. The level of lithium was measured according to Voynar method and expressed as mkq/ml. The results showed that during these periods (March, April, May), the inhibition of the epiphysis due to light factor could increase the level of lithium in blood and plasma. With prolongation of the light regime, this difference increases. This may continue up to 14 p.m. In contrast, keeping animals in darkness led to different results. The observed effects may be due to changes of metabolism following alteration of lithium level. These results may be useful to define the role of the epiphysis in the microelement metabolism.