The interplanetary magnetic field configuration on the ecliptic plane, within the inner heliosphere, has been compared to theoretical predictions based on the 1958 Parker's model. Taking into account that fast and slow wind have a different corotation radius, our analysis has shown that the overwinding of the spiral, which amounts to a few degrees as reported in the literature, can be largely reduced if we eliminate from the dataset those time intervals characterized by higher level of magnetic and plasma compression. Our results suggest that the presence of shocks and CME's alone is not a sufficient condition, as concluded by previous investigators, in order to justify the overwinding observed in the inner heliosphere. Plasma and magnetic field compressions, more generally speaking, have to be considered at the basis of this phenomenon. ¿ 1998 American Geophysical Union