Rhoads was to assist Castle, and they established a base in San Juan at the Presbyterian Hospital. Rhoads corresponded often with Simon Flexner at the Rockefeller Institute in New York regarding his research and career interests. In Puerto Rico, the Rockefeller group had more than 200 patients; historian and ethicist Susan E. Lederer notes that, while referred to as patients, they were primarily clinical subjects whose conditions were studied to advance medical research. Because of the effects of anemia and the suspicion that tropical sprue was related to diet, Rhoads experimentally controlled patients' diets. Lederer notes that in letters from this time, Rhoads referred to his patients as "experimental 'animals'." He wrote: "If they don’t develop something they certainly have the constitutions of oxen." Rhoads sought to experimentally induce the conditions he was studying in his patients rather than simply treat them. If they did develop tropical sprue, he could treat it with liver extract.