Pairs of miniature extraluminal force transducers were chronically implanted on the isthmus of 26 rabbits without disturbing reproductive function. No alterations were noted in the transducer-bearing animals in the numbers of corpora lutea, uterine implants, or viable conceptuses, or in fertility, pregnancy rates, rates of ovum transport, or peripheral plasma levels of progesterone (P) or estradiol (E2). Using these devices, isthmic motility was recorded in 14 unanesthetized rabbits throughout the 72 h following human chorionic gonadotropin injection (hCG) (75 IU) and coitus. This activity was comprised of distinct bursts (B) of intense contractile activity, interspersed with periods (IB) of relative quiescence. Although the basic pattern of B and IB periods persisted throughout this interval, significant changes were noted in the duration of its components. During this period, B durations increased, with all values greater (P less than 0.01) than estrus from 24 h following the ovulatory stimuli. IB durations diminished somewhat at 24 h, but were markedly prolonged at 60 and 72 h. The proportion of time spent in bursting activity was maximal and significantly greater than estrus (P less than 0.01) at 24 h, a time at which ova were retained at the ampullary-isthmic junction. This proportion was minimal, on the other hand, at 60 and 72 h, when ova were moving rapidly through the isthmus and uterotubal junction. Plasma P surged from estrus to a peak at 6 h, fell until 24 h, and gradually increased through 72 h. All post-stimulus P values were greater (P less than 0.01) than at estrus. The B and IB durations at 72 h were both positively correlated with the P levels at 6 h. These findings suggest the importance of specific patterns of isthmic motility in ovum transport and of P in its regulation.