Endogenous mechanisms of plant resistance to herbivorous insects could be adapted to enable the development of alternative strategies for pest control. Activation of the lipoxygenase (LOX) pathway by jasmonic acid, the hormone responsible for activating genes encoding protease inhibitors (PI), is one of the main direct defenses of plants against insects. In this study, soybean cultivars at the V3 stage that were either susceptible or resistant to herbivorous insects were used to investigate changes in the activity of LOX and concentration of PIs in response to 24- or 48-h feeding by fourth and fifth instar Anticarsia gemmatalis (Lepidoptera: Noctuidae) larvae. LOX activity (0.0-0.007 nm s-1/mg) and the PI concentration (60 to 125 mg of trypsin inhibitor / mg of protein and 90 to 120 mg of trypsin inhibitor / mg protein after 24 and 48 h after attack, respectively) both increased in response to damage caused by A. gemmatalis. Proteolytic activity decreased by approximately 50% in cultivars with different degrees of resistance. Raised enzyme profiles in the midgut of A. gemmatalis may be related to tryptic enzymes that increased, compensating for the inhibition of proteases by entomotoxic substances of the soybean cultivars.