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 find 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 Prot. 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 emissions are less intense. We present newly derived rotational periods for 15 G-type stars.