Objective To determine whether formation of the nucleolar channel system (NCS) in human endometrium depends on the presence of progesteronal steroids.Materials & Methods Tissues of late proliferative endometrium were obtained from 5 normally cycling women of reproductive age. Half of each tissue was cultured in the DMEM medium containing diethylstilbesterol (25 μg/mL) plus medroxyprogesterone acetate (25 μg/mL) (E+P culture). As a control, the other half was cultured in the medium alone. After 100 h incubation, the tissues were assessed for the formation of NCS with transmission electron microscope.Results NCS was observed in the endometrial epithelium treated with E+P or the medium alone. Moreover, giant mitochondria and glycogen accumulation were both seen in epithelia derived from both types of cultures.Conclusion Progesterone would be not indispensable for the formation of NCS in human endometrium. Transition of proliferative endometrium to the secretory stage in vitro could occur even in the absence of both estrogen and progesterone.