Ghrelin, along with several other hormones, has significant effects on appetite and growth in humans and animals. The aim of our study was to examine changes in relative growth rates, alpha- and beta-glucosidase activities and endocrine cell size in the midgut of 4th instar caterpillars of the pest insect Lymantria dispar L. after ghrelin treatment. Four subpicomolar injections of ghrelin (0.3 pmol) or physiological saline were applied every 24 h to two separate groups of fifteen caterpillars. Repeated administration of ghrelin in subpicomolar doses elevated the relative growth rate, induced alpha- and beta-glucosidase activities and increased the size of endocrine cells. The results are the first data about ghrelin effects on relative growth rate, digestive enzyme activities and midgut endocrine cells in insects. This information supports the use of this relatively simple model system in future studies of mechanisms underlying digestion in complex organisms.