In this study, the stability of methamphetamine, bromovalerylurea, acetaminophen and salicylic acid in formalin solution was investigated during 90 days. The stability of these drugs was examined in 4 types of formalin solution (10% and 20% concentration pH adjusted formalin (pH7.4) and each concentration pH unadjusted formalin solution) and at 2 temperature conditions (room temperature and cold storage).   Methamphetamine was very stable at cold storage in each formalin solution. In contrast, more than 80% of methamphetamine was converted into its N -methyl derivative, dimethylamphetamine in pH adjusted formalin at room temperature at 90 days. Bromovalerylurea was stable only when in pH unadjusted formalin at cold storage. Under the other conditions, bromovalerylurea was decomposed and observed a compound assumed to be the bromovalerylurea-formaldehyde reaction product by LC/MS. The concentration of acetaminophen was decreased gradually under all conditions and a compound assumed to be the acetaminophen-formaldehyde reaction product was detected by LC/MS and GC/MS. When salicylic acid was stored under all conditions, concentrations of salicylic acid did not change.   Thus, it appears that the stability of drugs varies individually in the presence of formalin and under different conditions (formalin concentration, pH and storage temperature). So, forensic scientists must note when attempting to determine the drugs in the formalin-fixed biological specimens.