BALB/cA females mated with BALB/cA males and exposed to DDK males showed 100% pregnancy block when exposure started at Days 0-3 of pregnancy. When the exposure to DDK males started at Days 4, 5, 6, and 7, the pregnancy rate was 33.3%, 50%, 90%, and 100%, respectively. (Hereafter, the BALB/cA females exposed to DDK males are referred to as the experimental females, and those exposed to other BALB/cA males as the control females.) Cell number of embryos at Day 3 of pregnancy from the experimental and control females after exposure starting from Day 0 was, respectively, 27.0 +/- 1.2 (mean +/- SEM) vs. 48.6 +/- 1.0 (p < 0.05) for the morulae and 34.8 +/- 1.3 vs. 64.1 +/- 2.0 (p < 0.05) for the blastocysts. Embryos from the experimental and control females after exposure starting from Day 3 or Day 4 were histologically examined after 24-h exposure. Poor development of trophoblast and inner cell mass was observed at Day 4 as general defects of the experimental embryos. In addition, about one third of the embryos were unexpectedly at the more advanced stages than the controls (abnormal overdevelopment). At Day 5, disintegration of the trophoblast layer and primitive endoderm was observed as common abnormalities of experimental embryos, but overdevelopment was not observed at this stage. Disturbance in trophoblast differentiation or failure of its maintenance may be the primary developmental defect displayed by embryos developing in a deteriorated uterine environment under the influence of the odor of alien males.