The morphometric changes (size of neurons and their nuclei) of protocerebral dorsomedial A1? and dorsolateral L2? neurosecretory neurons were analyzed in Lymantria dispar larvae after exposure to strong static (SMF, 235 mT) and extremely low frequency magnetic fields (ELF MF, 2 mT). Increase in the size of A1? neurons and their nuclei were observed after acute exposure to SMF. Decrease in the size of these neurons and their nuclei was observed after exposure to ELF MF. The size of L2? neurons and their nuclei tend to decrease after exposure to SMF and ELF MF. The quantification of protein bands within the Mr range corresponding to the large form of the prothoracicotropic neurohormone indicates that the amount of protein decreased after exposure to both types of magnetic fields.