Abstract: Effects of exogenous metallothionein (MT) on lymphocytes apoptosis, necrosis and mitochondrial membrane voltage (ΔΨm) of China Holstein cows were studied using 20 lactating cows in heat stress. They were randomly allocated to groups A, B, C and D. MT was supplemented at 0.0, 4.0, 8.0 and 16.0 mg/capita, respectively, by intravenous injection. Corrected milk yields in groups B, C, and D were significant higher than those in group A after being injected MT during days 51-60 (P<0.05); at 35 d, the lymphocytes apoptosis rate of group C (8 mg/capita) was significantly lower than those of groups B and D (P<0.05), but was not significantly different from group A (P>0.05). The lymphocytes apoptosis rate of group D (16 mg/capita) was the highest at 35 d (P<0.05) but significantly (P<0.05) decreased during days 50-60; Compared to 1 d, the lymphocytes necrosis rate of group A significantly (P<0.05) decreased at 50 d, group C (8 mg/capita) significantly (P<0.05) decreased at 35 d and 50 d, and began to show a rising trend at 60 d (P>0.05), while group D (16 mg/capita) significantly decreased at 50 d and 60 d. The mitochondrial membrane voltage (ΔΨm) of group A was the highest at 35 d, but the Δ Ψm of group B rose fastest at 35 d (P<0.05), and was significantly higher than at 20 d (P<0.05). The ΔΨm of groups A and B decreased at 50 d with that of group A falling fastest. The ΔΨm of group C was constant during days 35-50 but increased slightly at 60 d. The ΔΨm of group D (16 mg/capita) increased gradually during days 1-35 and remained constant during days 51-60. The lymphocytes ΔΨm and injected MT amount were positively correlated at 60 d. In conclusion, the heath of cows under heat stress can be improved, by exogenous MT injection. The injection of 8-16 mg/capita, can inhibit the lagged effects of southern summer heat stress in lactating cow.