Anatomy studies require cadavers to study the human body. Generally in Indonesia, the dead human body will be buried. This causes problems because the decomposition process of a cadaver that is preserved with formalin will be delayed and it causes environmental pollution. The toxicity of formalin can be reduced by neutralizing the formalin. This study aimed to compare the decrease of mice mass that were preserved with formalin then neutralized with sodium bicarbonate and those that were not neutralized. This study used 18 mice (Mus musculus) which were divided into 3 groups. They were the control group (not given preservative), group preserved with 4% formalin, and group preserved with 4% formalin then neutralized with sodium bicarbonate. All groups of mice were buried for 6 weeks. The changes in mass were assessed with an analysis of the percentage loss in mass. Based on the results of this study, the formalin group had a greater percentage of total mass reduction than the neutralize group. The formalin group had a higher decomposition rate than the neutralizing sodium bicarbonate group. The effectiveness of reducing the concentration of formalin is similar with neutralize group. Therefore, it can be concluded that 4% formalin is recommended for use to accelerate the occurrence of decay and decrease in mass.