The stability of drugs in formalin solution is an important factor in forensic investigation. Tissues (liver, lung, kidney, brain) taken from rats, which have been poisoned acutely with cocaine, were preserved in two different conditions, analyzed by GC-MS, and then compared. Organs of the first group were preserved and stored at - 20 degrees C without adding formalin, whereas the organs of the second group were preserved and stored in formalin solution at room temperature (25 degrees C). Serum samples were taken immediately after poisoning and studied as well. In specimens stored at - 20 degrees C, cocaine and its metabolite benzoylecgonine were detected in the tissues. Only benzoylecgonine was detected both in tissues and their formalin solution. It was observed that the distribution of cocaine in tissues had differed depending on the preservation conditions. The formalin solution in which benzoylecgonine was mostly detected was from liver. As a result, cocaine was detected in tissues stored at - 20 degrees C. It is recommended that both the formalin-fixed tissues and formalin solution should be analyzed concurrently to assure the accurate results (LOD = 3 ng/ml).