Average protonospheric electron contents, line integral measurements determined from observations of the ATS-6 satellite radio beacon at Hamilton, U.S.A. and Aberystwyth, U.K. are compared. It is shown that the two sets of data would be expected to be representative of measurements obtained at mid-latitude stations in North America and Europe respectively. The American protonospheric content magnitudes are lower than those from Europe and show a diurnal variation maximizing at night in winter through equinox in contrast to the daytime maximum found consistently in the Aberystwyth data. It is suggested that the differences may be interpreted in terms of ionospheric/protonospheric interactions in both local and conjugate hemispheres and the similarities in the forms of the diurnal variation of the American results with those of peak density in the conjugate ionosphere are noted.