We investigated the time-dependency of IVIM /non-Gaussian diffusion parameters in vivo and post-mortem using breast xenograft models (MDA-MB-231). fIVIM, perfusion-related fraction, dropped abruptly post-mortem, though not reaching 0 completely, suggesting imperfect modeling. fIVIM, estimated from the pseudo-diffusion model, was independent of the diffusion time. The significant decrease of sADC and ADC0 values and the increase in K values observed between short and long diffusion times, as well as after sacrification are in accordance with an increase in diffusion hindrance, as more water molecules hit many boundaries, such as cell membranes. However, different mechanisms may be at stake.