In the mammalian female reproductive tract, tachykinin neuropeptides, such as substance P (SP), are localized to a population of sensory fibers and their precise physiological role is still unknown. The aim of the present study was to characterize the population of tachykinin receptors in the pregnant rat uterus and to assess their regulation during the course of pregnancy and after delivery. The expression of the tachykinin NK₁ receptor (NK₁R), the tachykinin NK₂ receptor (NK₂R), and the tachykinin NK₃ receptor (NK₃R) in uteri from rats at different stages of pregnancy and on Day 1 postpartum was investigated by using a semiquantitative reverse transcription-polymerase chain reaction. The contractile effect of tachykinin receptor agonists acting selectively on the NK₁R, the NK₂R, or the NK₃R was investigated by conventional organ bath techniques. Serum levels of estrogen and progesterone were measured by RIA. Our data show that the expression and function of NK₁R and NK₃R varied along the course of pregnancy and at postpartum. Uterine NK₂R mRNA levels remain stable during the course of pregnancy and at Day 1 postpartum; and the contractions elicited by activating selectively the NK₂ receptor in the presence of the neutral endopeptidase inhibitor phosphoramidon (1 μM) were similar in early, mid, or late pregnancy. These results show that the expression and function of tachykinin receptors within the uterus vary with reproductive state and length of gestation, supporting a role for tachykinins in pregnancy and/or parturition in the rat.