Changes in fitness components including larval stage duration, relative growth rate (RGR), and mass of the gypsy moth, Lymantria dispar L. (Lepidoptera: Lymantriidae), were investigated in caterpillars fed a synthetic diet with or without a cadmiumsupplement (10, 30, 100, 250 Fg Cd/g dry food weight). Morphometric changes of large protocerebral dorsomedial A2 neurosecretory neurons, their nuclei and the electrophoresis profiles of brain proteins were analyzed in the 4th instar gypsy moths fed the examined diets. The duration of the fourth larval instars were prolonged and RGR and body mass reduced if the caterpillars were fed diets containing high concentrations of cadmium(100 and 250 Fg). The size of large A2 dorsomedial neurosecretory neurons and their nuclei were significantly higher in larvae fed the diets supplemented with 10, 100 and 250 Fg Cd. A large amount of neurosecretory material appeared in dorsomedial neurosecretory neurons in larvae fed diets with 100 and 250 Fg Cd. Differences in larval brain protein profiles in the region ofmolecularmass ranges (Mr) of 98kDa, 46kDa and 3.4-6.1 kDa were identified in the experimental groups.