The aim of this study was to analyze the function and expression of tachykinins, tachykinin receptors, and neprilysin (NEP) in the mouse uterus. A previous study showed that the uterotonic effects of substance P (SP), neurokinin A (NKA), and neurokinin B (NKB) in estrogen-treated mice were mainly mediated by the tachykinin NK₁ receptor. In the present work, further contractility studies were undertaken to determine the nature of the receptors mediating responses to tachykinins in uteri of late pregnant mice. Endpoint and real-time quantitative RT-PCR were used to analyze the expression of the genes that encode the tachykinins SP/NKA, NKB, and hemokinin-1 (HK-1) (Tac1, Tac2, and Tac4); and the genes that encode tachykinin NK₁ (Tacr1), NK₂ (Tacr2), and NK₃ (Tacr3) receptors in uteri from pregnant and nonpregnant mice. The data show that the mRNAs of tachykinins (particularly NKB and HK-1), tachykinin receptors, and NEP are locally expressed in the mouse uterus, and their expression changes during the estrous cycle and during pregnancy. The tachykinin NK₁ receptor is the predominant tachykinin receptor in the nonpregnant and early pregnant mouse and may mediate tachykinin-induced uterine contractions in the nonpregnant mouse. The tachykinin NK₂ receptor is predominant in the late pregnant mouse and is the main receptor mediating uterotonic responses to tachykinins at late pregnancy. The tachykinin NK₃ receptor is expressed in considerable amounts only in uteri from nonpregnant diestrous animals, and its physiological significance remains to be clarified.