Smooth muscle is known to be a malleable tissue in that it can adapt in vitro to maintain contractile capacity over a wide range of imposed lengths. Uterine smooth muscle in vivo is exposed to considerable mechanical stimuli during pregnancy that necessitates complex adaptive responses so that premature delivery of the fetus is not precipitated but that, at term, contractile effort is maximized. The molecular and physiological nature of any such responses is unknown but may involve plasmalemmal dense plaque-associated molecules acting as sensors and transducers of the extracellular mechanical stimuli into intracellular signals. Our aim, therefore, was to assess the form of gestational-dependent mechanoadaptation of uterine smooth muscle, in terms of active and passive length-tension profiles, and if it was associated with changes in expression and activation of dense plaque-associated proteins. Methods: CD-1 mice (non-pregnant and gestational days (D) 7, 13, 15, 17 and 19 , term d19) were humanely killed according to national guidelines, each the uterine horn weighed, the horns opened and the pups and placentas weighed, the pups humanely killed and uterine smooth muscle tissue processed for in vitro contractility measurements or Western blotting. Small uterine strips were bathed in physiological saline solution at 37°C, bubbled with 95% air-5% CO2 mixture and mounted in standard organ baths for isometric force measurement. Using a calibrated micrometer arm, tissues were initially set at slack length and passive tension noted, stimulated with high K PSS (60mM KCl substituted for NaCl) and active tension measured, washed in PSS and stretched by 10-20% and restimulated with high K PSS. The process was repeated to construct incremental passive and active length-tension curves. Tissue was probed by SDS-PAGE and Western blotting for α-actin, focal adhesion kinase (FAK), paxillin, c-Src and Erk1/2. Results: Pregnancy was positively correlated (Pearson’s correlation coefficient) with pup and uterine weight indicative of in vivo increased mechanical stimulation of the uterus. Equally, both the passive and active length-tension curves progressively shifted dramatically to the right. For example, at D19 maximum active tension was generated following 3.94+0.33-fold stretch beyond slack length compared to 1.91+0.12-fold for NP mice (mean+sem, p<0.05, Dunnets, n=4). Pregnancy was also associated with elevations in the expression and activation of FAK, c-Src and Erk1/2 relative to α-actin (which was invariant). Conclusion: These results suggest that alteration in the expression/activation of dense plaque-associated proteins may contribute to the physiological mechanoadaptation of uterine smooth muscle with pregnancy.