Mechanical models are important in many areas of medicine and physiological research. These are usually based on continuum mechanics using macroscopic mechanical parameters. However, knowledge of the microscopic structure of tissue, that is, the organization of structural proteins, gives useful information for improving such models for a given tissue. In this paper we image the structural changes in these proteins under strain by using multiphoton microscopy. This gives insight into the response to the tissue at microscopic level and can be used to modify existing mechanical models of tissue. Specifically we imaged the straightening of collagen fibers in bovine chordae tendinae and related this to the macroscopic strain applied to the tissue.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.