Measurements of ultrasonic velocity and absorption have been made in gaseous NH3 at 298.15°K, and show both rotational and vibrational relaxation to occur with approximately the same relaxation time. The estimated collision numbers for energy transfer are Zvib=Zrot= 4.9. The vibrational relaxation time of NH3 is considered to be abnormally short even though there is agreement between the experimental results and that calculated using the S.S.H. theory since the theory underestimates considerably the relaxation times of other small polar molecules including ND3. Approximate calculations of rotational relaxation on a classical “rough spheres” model give Zrot= 14.4. The discrepancy with experiment is attributed to the effect of dipole-dipole interaction in helping rotational energy transfer. The experimental collision numbers are significantly larger than published values calculated by the Mason-Monchick theory from observed transport properties. The paper includes an accurate experimental value for the velocity of sound in argon, which was used as a calibrating gas for the apparatus.