A solid-phase microextraction (SPME)-GC procedure has been developed for the analysis of four selected pesticides (propanil, acetochlor, myclobutanil and fenoxycarb) in water samples. Mass spectrometry (MS) was used and two different instruments, a quadrupole MS system and an ion trap operating in the MS-MS mode, were compared. A Carbowax-divinylbenzene SPME fiber was used. The performances of the two GC-MS instruments were comparable in terms of linearity (in the range of 0.1-10 microg/l in water samples) and sensitivity (limits of detection were in the low ng/l range); the quadrupole MS instrument gave better precision than the ion trap MS-MS system, but generally the relative standard deviations for replicates were acceptable for both instruments (<15%). Specificity with these two instruments was comparable in the analysis of ground water samples. Recovery tests were made to assess the applicability of the SPME procedure in the quantitative analysis of contaminated groundwaters.