Understanding of cellular interactions with a nanostructure requires tracking directly the nanostructure. Current investigation is challenged by the lack of a strong, intrinsic signal from the nanostructure. We demonstrate intensive four-wave mixing and third-harmonic generation signals from dimension-controllable silicon nanowires as small as 5 nm in diameter. The nonlinear optical signals observed from the nanowires are highly photostable with an intensity level of 10 times larger than that observed from silver nanoparticles of comparable sizes. This intrinsic optical signal enabled intravital imaging of nanowires circulating in the peripheral blood of a mouse and mapping of nanowires accumulated in the liver and spleen, opening up further opportunities to investigate in vivo cellular response to nanomaterials as a function of size, aspect ratio, and surface chemistry.