The integrated Hβ luminosities of giant H II regions and of H II galaxies can be predicted with accuracy comparable to the observational errors from the velocity widths of their emissionline profiles. In addition, giant H II regions also present a relation between linear size and their emission-line width. Based on the similarity of these relations with those expected for virialized systems, Terlevich and Melnick interpreted the observedsupersonic motions as due to the gravitational potential of a complex of gas and stars. We show that H II galaxies also present a relation between linear size and their emission-line width, which supports the gravitating model. We also show that the scatter in the luminosity-line width relation for H II galaxies seems to be correlated with linear size and compare the results with the “fundamental plane” for elliptical galaxies from Dressleret al. (1987).