The terahertz (~ 100 GHz to 10 THz) electrical properties of nanomaterials are of relevance both to the fundamental science of low-dimensional systems and to the operation of next-generation smaller and faster electronics. I will describe the first terahertz time-domain electrical measurements of single-walled carbon nanotube transistors. A ballistic electron resonance is directly observed with a picosecond-scale period corresponding to the roundtrip transit of an electron along the nanotube. The electron velocity is found to be constant and equal to the Fermi velocity, showing that the high-frequency electron response is dominated by single-particle excitations rather than collective plasmon modes. These results demonstrate a powerful new tool for directly probing picosecond electron motion in nanostructures.