Among the various types of urinary drugs and metabolites of forensic interest, those with an aromatic nitro moiety, an ester moiety, and amino-benzodiazepine metabolites were found to be readily decomposed, especially in samples severely contaminated with bacteria; however, the aseptic collection and storage of urine specimens has become possible by using a recently developed collection kit. Using the kit, the filtration sterilization was found to be effective in preventing the decomposition of various kinds of unstable drugs and metabolites. Its additional advantages over chemical preservatives include its ease of operation and not requiring any additives. The authors recommend using the kit in collecting urine specimens, because forensic specimens should be analyzed for "unknowns" that are often exposed to various temperatures when they are collected and transported. In order to evaluate preservation measures, methamphetamine, amphetamine, nitrazepam, estazolam, 7-aminoflunitazepam, cocaine, and 6-acetylmorphine were spiked into urine at 500 ng/mL each, and they were monitored for 6 months at 25, 4, and -20 degrees C, after the addition of NaN3 and/or filtration sterilization using the aseptic collection kit. 7 figures, 1 table, and 23 references