Objective: To observe the cellular damage of low-dose combined exposure to Hg, Pb and Cd on hippocampal neurons in rat. Methods: SH-SY5Y cells were randomly divided into 8 groups by 2×2×2 factorial design: control group, Pb exposure group, Hg exposure group, Pb+Hg exposure group, Pb+Cd exposure group, Hg+Cd exposure group and Pb+Cd+Hg exposure group. And the cell viabilities were measured. On this basis, an animal model was established. Twenty eight-week-old SD pregnant rats were randomly divided into four groups by random number table, and five in each group: the control group(distilled water), 1-fold metal mixture exposure group (1×MM, poisoning solution containing mercury chloride 0.15 mg/L, lead acetate trihydrate 25 mg/L, cadmium chloride 7.5 mg/L), 5-fold metal mixture exposure group (5×MM, poisoning solution containing mercury chloride 0.75 mg/L, lead acetate trihydrate 125.00 mg/L, cadmium chloride 37.50 mg/L), 10-fold metal mixture exposure group (10×MM, poisoning solution containing mercury chloride 1.50 mg/L, lead acetate trihydrate 250.00 mg/L, cadmium chloride 75.00 mg/L). Pregnant rats drank water until delivery. Twenty male pups were selected and exposed to these metals through breast milk until weaned. The heavy metals dose of poisoning water was adjusted, and then the weaned rats were exposed to heavy metals via drinking poisoning water until adulthood (postnatal day 83). The blood samples and brain hippocampus samples were collected to observe the ultrastructural changes of hippocampus, and to determine the levels of Hg, Pb and Cd in blood. In addition, apoptosis rate and fluorescence intensity of reactive oxygen species and intracellular free calcium concentration ([Ca²⁺]_i) in hippocampal neurons were measured. Results: Cellular factorial design analysis showed that Hg+Pb+Cd (at no observed adverse effect level, 1.0, 0.5 and 0.1 μmol/L, respectively)had a interaction on cell viability after 48 or 72 hours of combined exposure (P<0.05). The results of ultrastructure showed that mitochondria decreased, ridges and matrixes gradually dissolved in rat hippocampal neurons of 5×MM group; nuclear chromatin aggregated, more ridges and matrixes dissolved and the mitochondria also decreased in rat hippocampal neurons of 10×MM group. The concentration of Hg, Pb and Cd in the blood of 1×MM group, 5×MM group and 10×MM group were higher than those in the control group, and the differences were statistically significant (P<0.001). There was no significant difference in apoptosis rate between the 1×MM group and the control group. The apoptosis rate of 5×MM group and 10×MM group was higher than that in the control group, and the differences were statistically significant (P<0.001). There was no statistically significant difference in the fluorescence intensity of reactive oxygen species in hippocampal neurons of the 1×MM group and the control group. The fluorescence intensity of reactive oxygen species in the 5×MM group and the 10×MM group was higher than that in the control group, the difference was statistically significant (P<0.05). There was no significant difference in the fluorescence intensity of [Ca²⁺]_i between the 1×MM group and the control group. The fluorescence intensity values of [Ca²⁺]_i in the 5×MM group and the 10×MM group were higher than the control group, the differences were statistically significant (P<0.001). Conclusion Low-level combined exposure to Hg, Pb, and Cd caused synergistic neurotoxic damage, and the process may be related to the changes of neuronal apoptosis, reactive oxide species, and [Ca²⁺]_i levels.