To optimize the application of chemicals, a real-time variable rate system has been developed. The project incorporated an optical sensor that continuously measured plant height, cotton growth relationships in the MEPRT (MEPiquat Chloride Rate and Timing) software and a chemical rate controller into a single system. The plant height sensor was the continuation of a system initially evaluated in 1997. When used to predict the plant height in the field, the sensor was accurate a week after the pinhead square growth stage, and systematically overestimated the height at the first white flower. Pix application at the later date resulted in higher application rates than necessary. However, the height sensor was consistent in measuring consecutive passes through the field. Alternating passes of variable and uniform rate application were used. The average variable rate was higher in one field and lower in another compared with the applied uniform rate. This was due to the estimation of the average field height in both fields. Overall, the real-time system performed as it was designed.