  We present an analysis of the correlation between the mid-UV Mg II h and k
emission lines and measured rotational periods of G-type stars. Based on IUE
and HST high resolution spectra of a sample of 36 stars, we derive an
exponential function that best represents the correlation. We found that the
variation of the Mg II h + k fluxes is about a factor of 2.5 larger than that
of Ca II H+K, indicating that the UV features are more sensitive to the decline
of rotational period. The comparison of UV-predicted rotational periods with
those derived from empirical Prot - Ca II H+K flux calibrations are consistent,
with some scatter at large periods, where the emission are less intense. We
present newly derived rotational periods for 15 G-type stars.
