We use laser diffraction in the analysis of the transversal deformation that the papillary muscle of the female and male Wistar rat may undergo when is subjected to different tension (tension range, 0-30 mN) in the longitudinal plane. Papillary muscles from the right ventricle were illuminated at normal incidence with a He-Ne laser lasing at 594 nm at room temperature. The far-field diffraction pattern projected to a screen was recorded with a digital camera for its analysis. The analysis of the stress-strain curves from the two experimental groups shows that the papillary muscles from male rats exhibit a higher stiffness in the transversal axis compared to the female rats.