The way fluids flow into and fill nanotubes is of wide interest; however, there are currently no experimental data or validated theoretical models for this nanoscale process. We have carried out molecular dynamics simulations of carbon nanotubes imbibing oil at an oil/vapor interface at 300 K. We find that the smallest (7,7) nanotubes imbibe extremely rapidly (less than or equal to800 m/s) along the inner tube surface with the penetration length L a linear function of time. We derive expressions for the penetration length L and the velocity of the imbibing oil and relate both to the solid-fluid surface tensions and interfacial friction. The imbibition of oil by nanotubes is contrasted with the wetting of their external surfaces and that of planar surfaces. In all cases the appropriate surface tensions can be correlated with the minimum of the oil-molecule wall potential providing a means of predicting the internal and external wetting behavior of nanotubes. (C) 2004 American Institute of Physics.