The time for PGR application may correspond, in some areas where cotton is grown, to periods susceptible to rainfall occurrence. Thus, there is the possibility of the product to be washed without being absorbed by the plants. The use of an adjuvant may improve uptake efficiency and persistence of the product on the leaves in case of occurrence of rainfall after PGR application. This work had the objective to evaluate the effect of an adjuvant on the uptake efficiency of Mepiquat Chloride, as well as, the effect of the time for a 30 mm of rainfall occurrence on the necessity of re-application of the product. The results showed that the absorption of the Mepiquat Chloride is quick because even without adjuvant, about 50% of the product was absorbed in 45 minutes. The adjuvant improved the absorption of the product, especially 10 to 12 hours after the PGR application. After 24 hours there was no difference between the treatments with or without adjuvant, but it was possible to observe that even rainfalls happening 24 hours after the application of the product are able to wash product from the leaves, being necessary to apply again part of the rate initially used. The use of an adjuvant improves significantly the product absorption.