We present TAURUS Fabry-Perot velocity data in Hα emission of the disc of the grand design spiral M 100 (NGC 4321). We have studied the emission spectra of the 200 H ii regions most luminous in Hα, calibrated in luminosity using photometric Hα imaging from the literature. The emission spectra of individual H ii regions were fitted using one or more Gaussian functions, and analyzed in terms of their velocity dispersion. We concentrate on the question of whether the emission lines show values of their internal velocity dispersions of H ii regions which would be predicted from the virial theorem, and find that in general this is not the case. There is a systematic trend to super-virial line widths, characteristic of the non-equilibrium effects of powerful OB stellar winds, and supernovae. We propose that the lower envelope in velocity dispersion sigma , in the plot of Hα luminosity v. sigma represents the virialized regions, and give a tentative theoretical explanation for its slope of 2.6 in the log-log plane, in terms of density bounding for the regions of highest luminosity.