The adaptive growth of the uterus during pregnancy is a critical event that involves increased synthesis of extracellular matrix (ECM) proteins and dynamic remodeling of smooth muscle cell (SMC)-ECM interactions. We have previously found a dramatic increase in the expression of the mRNAs that encode fibronectin (FN) and its alpha5-integrin receptor (ITGA5) in pregnant rat myometrium near to term. Since the myometrium at term is exposed to considerable mechanical stretching of the uterine wall by the growing fetus(es), the objective of the present study was to examine its role in the regulation of FN and ITGA5 expression at late gestation and during labor. Using myometrial tissues from unilaterally pregnant rats, we investigated the temporal changes in Itga5 gene expression in gravid and empty uterine horns by Northern blotting and real-time PCR, in combination with immunoblotting and immunofluorescence analyses of the temporal/spatial distributions of the FN and ITGA5 proteins. In addition, we studied the effects of early progesterone (P4) withdrawal on Itga5 mRNA levels and ITGA5 protein detection. At all time-points examined, the Itga5 mRNA levels were increased in the gravid uterine horn, compared to the empty horn (P <0.05). Immunoblot analysis confirmed higher ITGA5 and FN protein levels in the myometrium, associated with gravidity (P <0.05). Immunodetection of ITGA5 was consistently high in the longitudinal muscle layer, increased with gestational age in the circular muscle layer of the gravid horn, and remained low in the empty horn. ITGA5 and FN immunostaining in the gravid horn exhibited a continuous layer of variable thickness associated directly with the surfaces of individual SMCs. In contrast to the effects of stretch, P4 does not appear to regulate ITGA5 expression. We speculate that the reinforcement of the FN-ITGA5 interaction: 1) contributes to myometrial hypertrophy and remodeling during late pregnancy; and 2) facilitates force transduction during the contractions of labor by anchoring hypertrophied SMCs to the uterine ECM.