Gossypol acts as a hormetic agent when ingested by Heliothis virescens larvae; low dosages stimulate growth, but high dosages retard growth. A mathematical model based on this effect more accurately predicts insect development than do earlier models based on a linear log-dose relationship. The weights of the larvae in each instar, independent of dosage, produce a bell curve centered at a given weight, and the average weight at a given dose is based only on the relative number of larvae in the different instars. Thus the large variation in weight noted at low to intermediate dosages can best be explained by the side range of instars present within a group. However, at high dosage most larvae remain in their early instars, and thus large variations in weight are not observed. A few larvae were unaffected at relatively high levels of gossypol, indicating genetic resistance to its effects. No significant mortality was observed at any dose level tested. All these effects can be explained if gossypol acts as a feeding stimulant or therapeutic agent at low dosages and as an antifeedant at high dosages.