Until now, the imaging of elastic fibers was restricted to tissue sections using the endofluorescence properties of elastin or histological dyes. Methods to study their morphology in vivo and in situ have been lacking. We present and characterize a new application of a fluorescent dye for two-photon microscopy: sulforhodamine B ͑SRB͒, which is shown to specifically stain elastic fibers in vivo. SRB staining of elastic fibers is demonstrated to be better than using elastin endofluorescence for

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... o-photon microscopy. Our imaging method of elastic fibers is shown to be suitable for simultaneous imaging with both other fluorescent intravital dyes and second-harmonic generation ͑SHG͒. We illustrate these findings with intravital imaging of elastic and collagen fibers in muscle epimysium and endomysium and in blood vessel walls. We expect SRB staining to become a key method to study elastic fibers in vivo.