We report the first study of coherent transients excited by ultrafast pulses of THz radiation. Using a newly developed optoelectronic source of well-collimated beams of subpicosecond pulses of THz radiation to excite N2O vapor, we have observed the subsequent emission from the vapor of coherent THz pulse trains extending to as long as 1 nsec. The origin of these subpicosecond THz pulses is a periodic rephasing, during the free-induction decay, of the more than fifty coherently excited rotational lines with commensurate transition frequencies. We term this new coherent phenomenon THz commensurate echoes. The echo repetition rate is equal to the frequency spacing between adjacent lines. From the decay and reshaping of the echoes the coherent relaxation time T2 and the anharmonicity factor for the N2O molecule are evaluated. © 1991 The American Physical Society.