As nitrogen management practices change to achieve economic and environmental goals, effects on weed-crop competition must be examined. Two greenhouse experiments investigated the influence of N amount and form on growth of maize and redroot pigweed (Amaranthus retroflexus L.).In Experiment 1, maize and pigweed were grown together in a replacement series (maize:pigweed ratios of 0 : 4, 1 : 3, 2 : 2, 3 : 1, 4:0) under three NHaNO3-N supplies (0, 110, and 220 mg N kg -~ soil). Maize was planted into established pigweed and plants were harvested 24 days after maize germination. Pigweed responded more to supplemental N than maize and accumulated 2.5 times as much N in shoots at the high N supply. Competition effects were not significant.Maize and pigweed were grown separately in Experiment 2 and supplied 220 mg N kg-l as either Ca(NO3) 2 or (NH4)2SO 4 plus a nitrification inhibitor (enhanced ammonium supply, EAS). In maize, EAS treatment did not affect shoot growth and reduced root growth 25% relative to the NO3-N treatment. In pigweed, shoot and root growth were restricted 23 and 86% by EAS treatment, respectively. Total plant N accumulation under EAS treatment was higher in maize, less in pigweed. Under EAS treatment, pigweed leaves were crinkled and chlorotic; leaf disks extracted in 70% ethanol, pH 3, contained less malate and oxalate but more NH4 compared to the NO3-N treatment. Maize leaf disk malate levels were generally higher compared to pigweed but were less due to EAS treatment. Ammonium level in maize leaf disks was unaffected by N form and EAS treatment increased oxalate levels. Final bulk soil pH was generally lower in pots where pigweed were grown and tended to be lower due to EAS. Leaf disk malate levels and soil pH were positively associated.Results indicate that pigweed is more likely to compete with maize when high levels of NO3-N are provided. Enhancing the proportion of N supplied as NHj-should restrict the growth of NH4-sensitive pigweed.