Data on the stability of drugs of abuse in oral fluid are needed to define the optimal transportation and storage conditions when this biological fluid has to be used for off-site screening and confirmation analyses, as well as in the preparation of reference material for external quality assessment schemes. The short-term and long-term stability of opiates, cocaine, amphetamines, and methylenedioxy derivatives in unstimulated oral fluid was evaluated in different storage conditions, with and without the addition of citrate buffer and sodium azide. Short-term stability was evaluated at 25 degrees C and 37 degrees C for up to 7 days. Long-term stability was evaluated at different time intervals for up to 2 months at 4 degrees C and -20 degrees C. The effect of three different freezing and thawing cycles was also studied. No significant loss of amphetamines and methylenedioxy derivatives, morphine, codeine, and benzoylecgonine was observed under any of the investigated conditions. Conversely, hydrolysis of the ester bonds of cocaine and 6-MAM, leading to the formation of benzoylecgonine and morphine, respectively, was observed under all the applied conditions when oral fluid was not buffered and preserved. The addition of citrate buffer (pH, 4) and sodium azide (0.1%) to oral fluid prevented their degradation during up to 7 days of storage at 25 degrees C and 37 degrees C and up to 2 months at 4 degrees C and -20 degrees C. The stability of the principal illicit drugs spiked in buffered and stabilized oral fluid is adequate for transportation of collected samples at ambient temperature and for shipment and storage of reference materials for external quality assessment schemes.