Variable-rate N application is needed to minimize excess N fertilizer without reducing yields. Tools recently developed to help producers know the correct N rate to apply have generally not been very reliable or easy to use at a production scale. However, advances in “active light”, reflectance sensor technology enable assessing N needs and automatically adjusting N application rates. The objective of this research was to evaluate the use of active light sensors for within-season N management on Missouri corn fields. In 2004 a study was conducted on seven producer fields in Missouri. The algorithm used for varying N was based on earlier plot work done in Missouri. Averaged over the seven fields, variable-rate application based on the active-light sensor measurements applied 37 kg/ha less than the average of the producer conventional rate. For one location, N applied using the sensor was much less (about 110 kg/ha) than the conventional rate, and this was the correct decision. This site had previously been managed as a pasture for several decades before being put back into row crop production in 2003. We presume that the dramatic decrease in N with variable-rate for this field was because significant soil N mineralization following the pasture. At two sites, sensor-based N application was greater than the conventional rate for the field. Over all locations, the economic impact of variable rate application was about a $10/ha increase in return compared to the conventional rate. We recognize that this amount alone will not convince farmers to adopt this innovative method of N management in corn. However, from this first year's results we are examining how soil and climate information might be used to adjust the algorithm for more profit. Based on these preliminary results, the use of active-light reflectance sensors to control in-season corn N fertilization is promising.