Prep plant growth regulator was applied to Pima S-6 cotton at rates varying from 0.75 to 3.0 lbs a.i./acre. Two combinations with Def and Accelerate were also applied. Four treatments resulted in significantly more yield of lint at the first harvest. Total yield for all treatments was not significantly different from the untreated plots. Boll opening and defoliation was significantly increased by certain treatments. No increase in green boll shedding occurred. There was little effect on fiber quality.