The accuracy of the Global Positioning System (GPS) satellite navigation system can be degraded by propagation effects on ray paths through the ionized atmosphere. The bulk of the plasma resides in the F layer, and models of total electron content have been developed to compensate for the effects of this ionization. However, use of the GPS system involves long ray paths through the tenuous hydrogen-based plasma of the protonosphere, and little is known about the electron content in this region. In the present study, the Sheffield University plasmasphere ionosphere model has been used to determine the electron content on the protonospheric section of GPS ray paths. Results are presented for stations at midlatitudes in the European and American sectors at both extremes of the solar cycle. The results are discussed in terms of the geometry of the flux tubes and the known behavior of the plasma. ¿ 1999 American Geophysical Union