Like other species with haemochorial placentation, pregnant rats show marked invasion of the uterine wall by trophoblast. While an endovascular pathway of invasion has been recognized for a long time, only recently, by application of cytokeratin immunostaining, the existence of an interstitial pathway of invasion has been established. Interstitial invasion is mainly effected by glycogen cell-like trophoblast arising from glycogen cell islands of the trophospongium opening up into the decidua, and from glycogen cell sheaths surrounding the intraplacental maternal arterial channels which are connected with the spiral arteries in decidua and mesometrial triangle. Quantitative evaluation of interstitial invasion in both maternal compartments was carried out on days 15-21, using PAS staining and cytokeratin and alpha-actin immunostaining for detecting trophoblast and defining maternal tissue compartments. Measurements of compartment size, cytokeratin-positive areas and invasion extent were performed using the KS400 image analysis system. A distinct pattern of interstitial trophoblast invasion emerged, starting from central decidual areas around the maternal arterial channels, and mushrooming into the mesometrial triangle reaching a peak at day 18, followed by gradual regression of the invaded areas. These measurements may serve as a basis for further experiments to evaluate factors which may influence the depth of trophoblast invasion.