It was there that he discovered his oratorical ability and his admiration for Left-leaning politicos, especially Marx and Engels; Reyes Barreiro leaned to the political Left for the entirety of his adult life.
("The citizenry of today is not what it was 30 years ago: it knows its obligations and its rights"—English Translation) He practiced medicine in Yucatán following his graduation for a short period before his socialist views got him into trouble with the government, and he was expelled from the state.
He spent many long hours making his rounds on horseback and, when possible, by carriage, often toting his doctor bag and several books on politics, poetry, science and medicine.
During the Mexican Revolution (1910–1920), Reyes Barreiro was often one of the few people allowed on the streets after dark during the curfew, to tend to his patients, revolutionaries, friends and foes alike.
On May 1, 1914, during the invasion of Veracruz by the American Frank Friday Fletcher, Reyes Barreiro was among the surgeons attending to the noted Mexican hero José Azueta.
Tenants grew increasingly agitated by the situation and decided to go on ‘‘strike’’ by ceasing rent payments, electing Reyes Barreiro as their leader.
Reyes Barreiro was a founding member of one of Veracruz' Freemason temples and, throughout his life in that city, wrote extensively for the newspaper El Dictamen.