Subsequently, he pursued Canon Law studies at the University of Valladolid, earning his bachelor's degree on March 30, 1669.

[1] On February 15, 1672, he entered the Colegio Mayor de la Santa Cruz in Valladolid by public examination and in 1672; he began to teach as extraordinary professor in the Faculty of Canons.

During this time, he lectured on the following topics: Furthermore, during this period he also presided over five concluding acts: During the years 1672 to 1679, he replaced various holders in his professorships.

[1] Pedro Antonio de Medrano initiated his legal career in 1686, temporarily replacing the judge of Vizcaya in the Chancery of Valladolid as interim, a position he held in the academic years 1686-1687 and 1688–1689.

In 1692, Licenciado Don Benito de Omaña served as interim Collegiate and Professor while Medrano was absent in Logroño, Haro, and other places to deal with matters of state and other proceedings.

[1] Due to the king's absence, Medrano was not appointed as interim viceroy of Navarre, a role that would have granted him full authority over the region.

Pedro Antonio de Medrano served three years as regent before being promoted to a position as an oidor in the Council of Orders in 1705.

[1][6] In 1705, Pedro Antonio de Medrano received the habit of Santiago and assumed his role in the Council of Orders.

His mother's coat of arms displays an azure field, an Or band, accompanied at the top and bottom by an argent tower, charged with gules.

His father Pedro de Medrano y Echauz was a knight of the Order of Alcántara, mayor of the Holy Brotherhood in Logroño in 1650 and ruler of Calahorra and its castle in the years 1673 and 1687.

In this letter, Tadeo informs the Duke of a military campaign in which Charles III of Spain led an army of over 14,000 men, including Turks and Moors, and recounts how he had the fortune to witness the first shots fired during the battle.