Two field experiments were conducted at Sids Agricultural Re­search Station during 1997 and 1998 seasons to study the response of cotton plant cv. Giza 80 to the application of plant growth retardant i.e., mepiquat chloride (0,100, 150, 200, and 250 cm³ /fed.), sprayed once at the start of flowering. The results revealed, that the application of mepiquat chloride reduced plant height and length of inter-nodes. tended to increase number of open bolls per plant significantly, in­creased seed cotton yield (in kentars / feddan), while seed index, lint percentage and lint micronaire reading and Pressley index were not sig­nificantly affected The application of mepiquat chloride exerted a signif­icant increase in phenol contents in leaves and oil content in seeds, while protein content in seeds was not significantly affected.