Transmission and scanning electron microscope appearances of epithelium in a pregnant uterine horn at and between the eggs, in a non-gravid horn of a unilaterally pregnant animal, and in the uterine horn of a pseudopregnant animal provided evidence of short-, medium- and long-range blastocyst effects. While cessation of mitotic division at 5 to 6 days post-coitum is solely the result of ovarian activity, the changes that occur in the shape of uterine epithelial cells apex seem to be due to the combined actions of ovaries and eggs. The development of apical club-shaped structures between 4 and 6 days, more pronounced in the pregnant horn compared with the pseudopregnant horn is apparently determined by ovarian and ovular effects. On the other hand, blastocysts induce the apparition of crater-cells at days 4 and 5 only in the pregnant horn : this could involve a mechanical or a chemical action of short- and medium-range egg effects. Similarly, on days 5 and 6, apical elongations are seen in the uterine regions occupied by the eggs, this finding points to a possible short-range blastocyst action.