Cotton plant is susceptible to different pests and deceases and is necessarily treated with pesticides. Insecticides are used most of all pesticides in agriculture worldwide. These chemicals are efficiently used; however they often cause the plant to be more susceptible to pests and deceases. The reason of this phenomenon is explained with the fact that defensive compounds in plant tissues lower. Herein we report the results on the effects of insecticides relating to three classes: carbophos (organophosphate), lannate (carbamate) and sumi-alfa (pyrethroid) on cotton leaf urease, phenylalanine ammonia-lyase activities and disulfide bonds amounts. Insecticides were sprayed with the concentration recommended by the producer against cotton pests. Field experiments were conducted on the cotton plant at the prebloom stage and leaf samples were taken on the 10th and 13th days of the treatment. Colorimetric analysis showed that urease and PAL activities in all samples treated with insecticides were lower than the control. Carbophos more strongly affected on the enzymes activities. Carbophos and lannate increased the amount of disulfide bonds, whereas pyrethroid sumi-alfa decreased.