Third Harmonic Generation (THG) microscopy is a non-fluorescent multi-photon technique that combines the advantages of label-free imaging with restriction of signal generation to the focal spot of the scanning laser. It allows three-dimensional imaging of refraction index mismatches and of hemoglobin. We applied it to image mouse tissues and to characterize blood vessels and blood flow velocity. In small arteries, THG line scanning reveals the pulse velocity curve and hence the heart rate.