Results are presented from an experiment to estimate the contribution of plasma on ray paths through the protonosphere to measurements of total electron content (TEC) using Global Positioning System (GPS) signals. Simulations using the Sheffield University plasmasphere ionosphere model show that observations of GPS satellites made at two stations separated by a few degrees of latitude could involve a common ionospheric volume but very different intersection geometries of the ray paths with protonospheric flux tubes. Experimental results demonstrate that, on average, higher equivalent vertical TECs are measured on ray paths to the south than those to the north of the European midlatitude stations considered here. The observations are discussed in terms of the known asymmetries of the protonospheric flux tubes, and caution is advised in the use of thin-shell ionospheric models for precise determination of TEC or correction for its effects on GPS systems. ¿ 1999 American Geophysical Union