Antifertile effects of PGF2 alpha, PGE2, PGE1, sulprostone and other PGs were evaluated in different pregnancy models in rats, guinea pigs and rhesus monkeys and the underlying mechanisms of action were investigated. Quantitative and qualitative species differences and pregnancy stage dependency were recorded. Basic regulatory differences of the pregnant uterus seem to exist in these species. In early pregnant rats, abortifacient effects were based on luteolytic effects, independent of the PG used. The myometrium was found to be refractory to the injected PG as long as serum progesterone levels were kept high. By contrast, in guinea pigs after the luteoplacental shift of progesterone secretion (tested after day 40 p.c.) and in rhesus monkeys even before this shift (tested day 20 p.c.) abortifacient effects were found to be exerted by direct stimulation of the myometrium. Uterine stimulation was possible in the presence of any level of serum progesterone. The induction of uterine PG synthesis was probably of importance supporting the expulsion. The role of obvious tissue damage within the conceptus remained uncertain. In contrast to rats there seems to be a pre-existing PG-sensitivity of the pregnant myometrium in guinea pigs and primates. In guinea pigs sensitivity slightly increased for E- but not for F-type PG toward term. Oxytocin sensitivity was found to increase by a factor of more than 100 between days 23-63 of pregnancy. Time dependent changes in uterine receptivity to PG and oxytocin may be considered as a regulatory principle which might permit parturition to occur in the presence of progesterone as an evolutionary adaptation to a placental progesterone secretion which cannot be abolished. It was concluded that in the presence of already established gradual uterine responsiveness to PG (and Oxytocin) during gestation efficient blocking mechanisms for uterine PG-formation must exist in order to explain uterine quiescence. Almost complete resistance of pregnancy to oestrogen which exists in humans, monkeys and guinea pigs was considered as to be pharmacological evidence of such a mechanism. The principles of endocrine control of the myometrium and its pharmacology seem similar in guinea pig and primate pregnancy. The guinea pig might therefore provide a relevant model to study potential drug effects on the regulatory balance of the pregnant uterus and also to achieve a better understanding of human uterine physiology.