The speed of sound has been measured in nitrogen on four isotherms at T/K = 250, 275, 300, and 350, and at pressures between 0.1 MPa and 30 MPa. The measurements were made using a spherical resonator operating at frequencies between 5 kHz and 26 kHz where the speed of sound in nitrogen differs significantly from the corresponding zero-frequency value because of the exceptionally long vibrational relaxation time exhibited by this gas. Corrections for this effect have been calculated. The final results have an estimated uncertainty of less than +/-0.01 per cent. The performance of the spherical resonator at high pressures is also discussed.