A significant portion of the NO₃ from agricultural fields that contaminates surface waters in the Midwest Corn Belt is transported to streams or rivers by subsurface drainage systems or “tiles”. Previous research has shown that N fertilizer management alone is not sufficient for reducing NO₃ concentrations in subsurface drainage to acceptable levels, therefore additional approaches need to be devised. We compared effectiveness of oat and rye cover crops for reducing NO₃ concentration and load in subsurface drainage water for a no-till corn (Zea mays L.)-soybean (Glycine max [L.] Merr.) management system. A winter rye (Secale cereale L.) cover crop was drilled after harvest over the entire plot area each year and then chemically killed before planting the following spring. A spring oat (Avena sativa L.) cover crop was broadcast seeded into the standing corn and soybean crops near physiological maturity. The oat cover crop treatment winter-killed and did not need to killed in the spring. Twelve 30.5 x 42.7-m subsurface-drained field plots were established in 1999 with an automated system for measuring tile flow and collecting flow-weighted samples. The rye treatment and a control were initiated in 2000, the oat treatment was initiated in fall 2005, and all treatments were replicated four times. Neither the rye nor oat cover crop treatments significantly reduced cumulative annual drainage water flow in any year. Flow-weighted NO₃ concentration of drainage water was reduced in 4 out of 5 years by the rye cover crop and in 2 out of 5 years by the oat cover crop. Averaged over 5 yr, the flow-weighted NO₃ concentrations were reduced by 48% by the rye cover crop and by 25% by the oat cover crop. Nitrate loads in the drainage water were significantly reduced by the rye cover crop in 1 out of 5 years and in no years by the oats. It seems that historically high cumulative annual drainage and high variability among replications hindered detection of significant treatment differences for NO₃ load. A rye or oat cover crop grown after both corn and soybean have the potential to reduce the NO₃ concentrations in water delivered to surface waters by subsurface drainage systems.