Excitation functions of (p,n) reactions have been determined for proton energies between 100 and 400 MeV using targets of Sc45, Ni64, Rh103, Te130, and W186. The energy dependence of the cross sections was found to vary from E-1.31 to E-0.75 over this range of mass numbers. This variation is apparently related to the increase in attenuation of the incident and outgoing nucleon waves with increasing target mass number. The experimental cross sections were compared with Monte Carlo cascade calculations based on a simple model which did not take refraction of the incident protons into account and which made use of a square-well nuclear potential. Serious deviations were observed between the calculated and the experimental energy dependencies, indicating the necessity of employing a more sophisticated model than one based on simple intranuclear nucleon-nucleon cascades.