The aim of this study was to characterize the ability of late-pregnant (days 15-22) rat uterine tissue rings to contract in response to electric field stimulation in vitro. For this purpose, maximum rhythmic contractions were elicited by optimum choice of the period time and the pulse width, the two main parameters of electric field stimulation. In parallel, the plasma 17beta-estradiol and progesterone levels were determined. It was found that the contractility ratio, i.e. the quotient of the optimum pulse width and the period time, is a good parameter with which to express the contractility. The larger the contractility ratio, the better the ability to contract. Evaluation of the area under the curve did not furnish information relating to the contractility in this method. A very close correlation was observed between the contractility ratio and the quotient of the 17beta-estradiol and progesterone levels on different days, demonstrating that the in vitro ability characterized by the contractility ratio is in keeping with the physiological regularity. There was also a very close correlation between the contractility ratio and the quotient of the alpha1- and beta-adrenergic receptors, suggesting the main role of the numbers of alpha1-receptor in pregnant uterine contractility. It is believed that this is the first in vitro model to give a numerical measure concerning the ontogeny of uterine contractility in late pregnancy.