A cytometric analysis was made on the adult adipose tissue of sectioned abdomens of normal and experimentally treated adult male Drosophila melanogaster. The cell size and proportions of reserve substances in the fat cells did not vary significantly over a 1-week period from 10-19 days of age. Ring glands transplanted from late third-instar, male larvae into male adults caused no significant changes in cell size and proportion of lipid in the male host cells. However, corpus allatumcorpus cardiacum complexes transplanted from adult D. melanogaster or D. virilis females into adult males caused the host fat cells to increase in size about 1.3 times and the amounts of lipid and glycogen deposits became significantly larger. Similar effects were obtained using corpora allata from D. virilis females. Synthetic juvenile hormone injected into males caused a 1.7-fold increase in cell size and the amounts of lipid and glycogen deposits increased significantly. © 1969.