A study of the High Plains aquifer in Okla homa was initiated in 1996 to: (1) provide the information needed by the Oklahoma Water Resources Board to manage the quantity of water produced from the aquifer; and (2) provide base line water-chemistry data. The approach used to meet the first objective is to develop a digital ground-water flow model. The model will be cali brated, in part, by comparing simulated and esti mated predevelopment discharge from the aquifer to streams and cross-boundary flow. This report presents the estimated predevelopment discharge to streams from the High Plains aquifer. Streamflow data were the primary source of information used to estimate predevelopment dis charge from the High Plains aquifer. Data from 30 streamflow stations between the Arkansas and Canadian Rivers were considered in the analysis, and winter low-flow frequencies for 7-, 14-, and 30-day periods were determined for 25 stations. The 14-day low flow with a recurrence interval of 2 years was the primary value used to estimate pre development discharge from the aquifer. The streams that drain the eastern part of the High Plains aquifer in Kansas (generally east of 99.5 longitude) are estimated to have had large predevelopment discharge from the aquifer, and most of them received discharge from near their headwaters. For streams with more than one streamflow gage, the upper perennial reaches appeared to have gained more discharge from the aquifer than the lower reaches. The total predevel opment discharge from the aquifer in this area to several streams is estimated to have been about 312 cubic feet per second, not including discharge that probably went directly to the Arkansas River. The Cimarron River and its tributaries are estimated to have gained about 78 cubic feet per second, but nearly one-half that amount was lost in the lower reaches of the river. The cause of the loss in the lower reaches is unknown. The Beaver River and its tributaries are estimated to have gained a net of about 10 cubic feet per second above Wolf Creek with the upper reaches gaining more than the lower reaches. Wolf Creek is estimated to have gained 30 cubic feet per second over its total length.