[Objective] This study aimed to research the effect of metal othionein on cellcycle, apoptosis rate and subsets distribution of lymphocytes in peripheral blood of dairy cows under heat stress, so as to perfect the regulative mechanism re-searches of MT to anti-heat stress. [Method] Twenty lactating Chinese Holstein cows were randomly divided into four groups (A, B, C and D), and injected with 0, 4.0, 8.0 and 12.0 mg Zn-metal othionein, respectively by intravenous route. Blood sam-ples were col ected at 1st, 16th, 31st, 46th and 61st day, and the dynamic changes of cellcycle, apoptosis rate and subsets distribution of lymphocytes were determined. [Result] The apoptosis rate of cells in group B and C was lower than those in group A by 26.63% (P>0.05) and 24.84% (P>0.05) respectively. The number of cells in the G0/G1 phage in trial groups was increased and the number of cells in the S and G2/M phages tended to decrease, but there were no significant differences (P>0.05). The number of CD3+ T cellin three trial groups was greater than those in group A by 7.02% (P>0.05), 5.45% (P>0.05) and 3.85% (P>0.05) respectively, while the number of CD4+ T cellin trial groups was higher than those in control group by 31.04% (P