FillOpaQ is a filler developed in the ErasmusMC Rotterdam to fill and identify blood vessels. The low viscosity of this fluid, and the short hardening time (8-10 minutes) makes this fluid especially suitable for research on small vessels like arterioles and venules. The hardening reaches a rigidity level comparable to rubber, which makes this material especially appropriate for dissection of these vessels. The fluid can easily be made radio-opaque, which makes it even more interesting for research and education purposes.