The vacuum polarization potential is included in the interaction Hamiltonian for 1 S − 2 S excitation of the hydrogen atom by electron impact. The excitation amplitude calculated field theoretically is found to be lowered by 0.47 t 2 / ( t 2 + 9 ) 3 where t 2 = 4 | P − Q | 2 , P and Q being the momenta of the incident and scattered electrons respectively.