This paper describes the method development for the determination of ultra-low level pesticides from high-elevation lake waters by large-volume injection programmable temperature vaporizer (LVI-PTV) GC/MS. This analytical method is developed as a subtask of a larger study, background of which is summarized in a poster presentation Abstract # 1220-1 1, Tuesday, March 11, 2003. Compounds of interest include organophosphorus, organochlorine, carbamate, synthetic pyrethroid, and sulfonic acid pesticides as well as triazine, aniline, thiocarbamate, amide, phthalate, and substituted urea herbicides. We anticipate the amounts of pesticides in our study sites to be lower than the detection limits of conventional analytical techniques (i.e., pulsed-splitless GC/MS). Our approach to achieving lower detection limits is to extract analytes from a large volume of water and analyze a substantial fraction of the extract using LVI-PTV- GC/MS. This approach should result in a I00-fold increase in the amount of pesticide reaching the detector, compared with previous studies of pesticides in the Sierra Nevada. The large- volume sampling and extraction procedures are the subject of a companion paper at this conference (Abstract # 1220-1 1). This paper describes optimal LVI-PTV-GC/MS conditions, presents analytical figures of merit of the method, and compares its performance with that of GC/MS with conventional pulsed-splitless sampling. A pulsed-splitless GC/MS method was first developed and optimized for use as the reference for the LVI-PTV-GC/MS method development and optimization. The former method is also used in the analysis of the extracts produced during the extraction method development. For the same total amount injected (10 pg), preliminary results of the LVI-PTV-GC/MS method (20-uL injection of 0.5 pg/uL pesticide standard mix) generally show analyte responses at greater than I00% higher than the responses produced by the pulsed-splitless GC/MS method ( I -uL injection of 10 pg/uL pesticide standard mix). Currently, limits of detection for the pulsed- splitless method are in the range of 0.6-15 pg/uL for the analytes of interest. These experiments were carried out on an Agilent GC 6890A and 5973N MSD with a Gerstel MPS-2 autosampler and CIS 3 injector.