We analyse the data accumulated on the description of the uterine kallikrein-kinin system in estrous cycle, pregnancy, pseudopregnancy and hormonal supplementation, and discuss the possible role of these findings in relationship to the vasoactive changes of gestation. We conclude that the evidence supports a participation of kallikrein in implantation and maintenance of uteroplacental blood flow. A decreased urinary kallikrein in women in conditions of an impaired uterine blood flow might be related to a deficient response of this vasodilator system.