Mechanical load regulates ventricular growth, function, and structure from the earliest stages of cardiac morphogenesis through senescence. Dramatic changes in cardiac form and function have been defined for developing cardiovascular systems, and changes in mechanical loading conditions can produce structural malformations such as left heart hypoplasia. To date, relatively little is known regarding the interactions between changes in mechanical load, morphogenesis, and the material properties of the embryonic heart. We tested the hypothesis that passive material properties in the embryonic heart change in response to altered mechanical load and that microtubules play an important role in this adaptive response. We measured biaxial passive stress-strain relations in left ventricular (LV) myocardial strips in chick embryos at Hamburger-Hamilton stage 27 following left atrial ligation (LAL) at stage 21 to reduce LV volume load and create left heart hypoplasia. Following LAL, myocardial stresses at given strains and circumferential stiffness increased versus control strips. Western blot analysis of LAL embryos showed an increase in both total and polymerized beta-tubulin and confocal microscopy confirmed an increase in microtubule density in the LV compact layer versus control. Following colchicine treatment, LV stresses and stiffness normalized in LAL specimens and microtubule density following colchicine was similar in LAL to control. In contrast, Taxol treatment increased myocardial stresses and stiffness in control strips to levels beyond LAL specimens. Thus, the material properties of the developing myocardium are regulated by mechanical load and microtubules play a role in this adaptive response during cardiac morphogenesis.