He was deputy to the Estates-General of 1789, for the nobility, and was one of the first aristocrats to renounce his privileges on the night of 4 August 1789. He continued to serve in the National Assembly and National Constituent Assembly and in January 1791 repaid to the Treasury the 60,000 francs it had cost Louis XVI to provide him and his brothers with an education at the École Militaire. In November 1790 he fought a duel with the Duc de Castries. The duke wounded him and it was briefly feared that he had tipped his sword with poison. Lameth was so popular that a mob stormed Castries' house in revenge. As the Assembly began to divide into factions, Lameth, a constitutional monarchist, was identified with the Feuillants and he was arrested in Rouen on 12 August 1792 for protesting against the Attack on the Tuileries. Since the French Revolution moved toward a Republic, he emigrated to Hamburg.