Apparatus for examining a fluid-containing porous sample, by a combination of centrifuging to move fluid into, out of, and/or within the sample and NMR to monitor the amount of fluid present at locations within the sample has a magnet system to provide a magnetic field; and a centrifuge rotor comprising a holder (18) for the sample and mounted to move the sample within the magnetic field. The apparatus preferably includes gradient coils (16, 116) superimposing a magnetic field gradient onto a field provided by magnets (11, 12, 111, 112). At least one radio-frequency coil (20) is located on the centrifuge rotor in a position 10 which surrounds the sample in the holder. The coil axis is transverse to the magnetic field and field gradient when measurement is made. Consequently NMR measurement of the distribution of fluid in the sample can be made while the centrifuge is in motion.