In toto analysis of cholinesterase activity in the cephalic extremity of 9- to 14-day old mouse embryo permits to define four different stages. Stage 1 corresponds to activity in the cephalic neural crest-derived cells. Stage 2 is characterized by activity in the proximal part of both first branchial arches and in the oculomotor blastemata. Stage 3 corresponds to a decrease of the activity, which appears in the area of fusion of several facial processes. Stage 4 corresponds to activity in nervous and muscular blastemata. Cholinesterase activity is thus chronologically varying and implies several differentiating cell populations.