Many organophosphate pesticides (OP) such as phosphamidon are unstable in aqueous solutions and especially in blood in the presence of esterases. In a case of intoxication, the phosphamidon concentration in serum decreased from 10 mg/L to 4.4 mg/L after storage at -20 degrees C for six months; nearly complete degradation was observed after three years. Dimethyl phosphate (DMP) is a metabolite of phosphamidon, mevinphos, dicrotophos, monocrotophos, dichlorvos, and trichlorfon. A gas chromatographic-mass spectrometric method with deuterated DMP-d6 as internal standard for the determination of DMP in biological material was validated. DMP was found in all of the patient's samples (3.9 and 4.9 mg/L in blood, 33.5 and 50.4 mg/L in urine, and 8.1 mg/L in gastric fluid), even after storage at -20 degrees C for up to 3 years. No hints for a degradation of DMP when spiked in fresh blood and stored at 4 degrees C for 1 week and stored in water over a time period of 10 months. Looking for the stable metabolites like DMP in cases of suspected OP intoxication is recommended.