Staining with paraldehyde-thionin-paraldehyde-fuchsin labeled 10–14 median neurosecretory cells (MNC) in each hemisphere of the brain of diapause and nondiapause adults of the blow fly, Protophormia terraenovae (Diptera: Calliphoridae). Various surgical operations were performed on the brain-retrocerebral complex of female adults to investigate the role of these cells in ovarian development and diapause. Under the diapause-averting conditions of LD 18:6 (18 h light and 6 h darkness) and 25°C, most of the intact and sham-operated females had vitellogenic ovaries, whereas all of the females with the MNC completely removed had immature ovaries. The corpus allatum (CA) was significantly smaller in the MNC-removed females than in the intact and sham-operated ones. After severance of the nervi corporis cardiaci, most of the females had immature ovaries. Even after severance of the cardiac recurrent nerve, there were vitellogenic ovaries in about 60 % of the females. However, after removal of the corpus cardiacum and hypocerebral ganglion complex (CCHG), no females had vitellogenic ovaries. Under the diapause-inducing conditions of LD 12:12 and 20°C, most females had immature ovaries and the CA was small irrespective of the surgical procedures. The results indicate that the MNC may secrete an allatotropic factor to stimulate vitellogenesis. The factor seems to be released to the hemolymph from the CCHG and a part of the aorta.