 From 1965 to 1967, he served in the United States Air Force as a doctor at Andrews Air Force Base near Washington, D.C.In 1967, Phelps returned to Iowa City to work for a year as an NIH postdoctoral fellow under Mansour F. Armaly in the University of Iowa's glaucoma research laboratory, an experience that persuaded him to become a glaucoma specialist.He continued at the university as a resident in ophthalmology from 1968 to 1971.