The ultracytochemical localization of the enzyme acetylcholinesterase (AChE) was studied in the germinal epithelium and cortex during chick ovarian morphogenesis at the 7, 12.5, 14 and 19 day of incubation. The results evidenced at day 7 the presence of the enzyme in some somatic cells, both "dark" and "light" and in some gonocytes. The reaction appears also in some tracts between these types of cells. At day 12.5 the reaction is present in some somatic cells only towards the deepest zone of the cortex, in many oogonia and oocytes and in some tracts between germ and somatic cells. At day 14 and day 19 the various cell categories of the cortex are negative for the reaction. The significance of the presence of the enzyme is discussed in relation to an embryonic cholinergic system active during morphogenesis. Regarding the presence of the enzyme in germ cells, the positivity in endoplasmic reticulum cisternae associated to mitochondria may suggest an implication of the enzymatic activity in the proliferation and transformation of these organelles.