The feasibility of using black polystyrene microspheres as surrogate ova to obtain quantitative information regarding ovum motion in the oviductal isthmus was tested in rabbits. The 175-microns diameter surrogates were instilled into the ampulla at the time of ovulation, and later the oviducts were exposed for direct observation of surrogate movements in the isthmic lumen. With the aid of transillumination, the black spheres were visible through the intact tubal wall and cinematographic records were made to quantify their movements. The frequency of surrogate movements was 5/min at 24, 36 and 48 h after coitus an 10/min at 60 and 72 h. The motion was typically oscillative with individual movements less than 4 mm in length; usually no net displacement occurred during 1-2 h of observation. These preliminary data, when used to test the stochastic model of ovum transport proposed by Verdugo et al. (1980), predicted isthmic transit times of only a few hours. Thus, the long duration of ovum residence in the rabbit isthmus may not be due to a slow transport process governed by random motion.