The effect of stressful stimuli on uterine motility during pregnancy and parturition was studied in sheep and rabbits. The effects of epinephrine and various alpha- and beta- adrenergic blocking agents were also investigated. By comparing the results of these experiments, the authors conclude that the increase in epinephrine level (stress response of the organism) is the direct cause of the observed changes in uterine motility. Both stress and epinephrine caused either an activation or an inhibition of uterine motility. The direction of the effect depended on the ratio of sex-steroid concentrations in the plasma. Blood plasma levels of oestradiol-17β and of progesterone were determined in the sheep. Both stress and epinephrine inhibited uterine motility only when plasma levels of oestradiol-17β were very high (oestrogen-domination). The biological relevance and clinical implications are discussed.