Mature ovarian eggs of Drosophila can be activated by treatment with hypotonic buffers. Well-fed, 4-day-old virgin flies contain large numbers of partially dehydrated mature eggs. When these eggs are transferred to a hypotonic culture medium, the ovarian eggs swell immediately and within minutes up to 70% become impermeable. The following cellular events ensue: meiosis, which had been arrested at metaphase I, is completed and the "polar body" nuclei fuse; cortical multivesicular bodies with acid phosphatase activity appear within minutes; polar granules fragment, dissociate from mitochondria, and become associated with polysomes; finally, monosomal ribosomes move to the polysomal region of a sucrose gradient. Each of these events corresponds to the normal in vivo effects of ovulation of oocytes, whether or not they are fertilized. When ovarian eggs of a parthenogenetic strain of D. mercatorum were activated by hypotonic treatment, some eggs developed into normal embryos. The presence of high potassium, low pH, and polyethylene glycol enhanced the frequency of normal development. Thus, we suggest that the rehydration of mature oocytes, as they move from the ovary to the uterus, activates the maternal program of the oocyte.