De Thou returned to Paris in 1572, and meaning to take orders, applied himself principally to the study of Greek and of the canon law. In the next year he visited Italy in the train of Paul de Foix, ambassador of France to the Pope and other Italian sovereigns, and employed himself diligently and profitably in cultivating the acquaintance of learned men, and in collecting materials for his history, the design of which he had already conceived. He returned to Paris in 1575, and during four years applied himself chiefly to study, taking various occasions to extend his travels into Flanders and Germany. In 1578 he was appointed Conseiller-clerc to the parliament of Paris, and in 1581, one of a commission sent into Guienne, to provide for the better administration of justice, which had been greatly impeded by religious dissension. Returning to Paris in November, 1582, immediately after the decease of his father, and having become the head of his family by the death of two elder brothers, he determined to abandon the ecclesiastical profession, and exchanged his place of Conseiller-clerc, for the lay appointment of Maître des Requêtes. In 1586 he obtained the reversion of the office of Président à Mortier, held by his uncle Augustin de Thou; and having obtained a dispensation from the ecclesiastical engagements which he had contracted, he married, in 1587, Marie de Barbanson. When the Parisians embraced the party of the League, in 1588, and Henry III. was obliged to quit the capital, De Thou followed the person and fortunes of the monarch, and received a commission to travel through Normandy and Picardy, to sound the intentions, and, if possible, to secure the adherence of the authorities, civil and military, of those provinces. His services were rewarded by the dignity of Conseiller d’État. In the autumn he was present at the convention of the States at Blois; but he returned to Paris before the murder of the Duke of Guise. He was not informed of the intention to commit that crime; and he believed, from certain peculiarities of behaviour, that the king had sent for him expressly to communicate that intention, but had changed his mind during the course of the interview. In the tumults which took place on the arrival of the news at Paris, De Thou’s life was in considerable danger, until he effected his escape under the disguise of a soldier, and returned to Blois.