The luminal and glandular epithelia of the rat uterus during delayed implantation have been studied with the electron microscope. The tall columnar cells of the luminal epithelium are characterized by a distinct zonation of organelles. On the free surface there are short microvilli and occasional cytoplasmic extrusions, with terminal bars joining adjacent cells at their apical margins. Beneath the free surface are numerous pinocytotic vesicles and mitochondria. Above the nucleus are a prominent Golgi apparatus with large dilated vacuoles, and numerous dense inclusion bodies. Large clusters of lipid droplets and aggregations of mitochondria occupy the basal zone of the cell.

The apical portion of the gland cell has fewer pinocytotic vesicles and the Golgi is smaller than in the luminal epithelial cells. Dense bodies are numerous above the nucleus. The basal portion of the gland cell is devoid of lipid, but there are often one or more enlarged mitochondria. Although a dense PAS+ substance fills the lumina of the glands, there is no obvious evidence of secretory activity in the glands during delay.

The relationships of the epithelia to the environment of the blastocysts during delay is discussed.