[2] That same year he accompanied Christian August, Prince of Waldeck and Pyrmont, Field Marshal of the Portuguese land army, to Lisbon.

However, after Prince Christian died in 1798, he set up a private medical practice, and subsequently accepted the post of surgeon to English troops in Portugal.

He requested to join the scientific crew of the First Russian voyage of circumnavigation but received a polite rejection letter; the expedition's ships would take on board the official naturalist at Copenhagen.

He implored with ambassador to Japan Nikolai Rezanov, and supported by Captain Adam Johann von Krusenstern, his petition to join the expedition was granted.

However he was able to publish his findings in books such as Plantes recueilles pendant le voyage des Russes autour du monde: expédition dirigée par M. de Krusenstern.

In 1821 he proposed to the Tsar Alexander I and to the Academy of Sciences to lead an ambitious exploratory and scientific expedition from São Paulo to Pará, in the Amazon, via a fluvial route.

In March 1822, he returned to Rio in the company of scientists Édouard Ménétries (1802-1861), Ludwig Riedel (1761-1861), Christian Hasse and Nester Rubtsov [pt] (1799-1874), who would take care of zoological, botanical, astronomical and cartographical observations during the expedition.

With the aim of illustrating and documenting his findings, the Baron hired painters Hércules Florence, Johann Moritz Rugendas and Adrien Taunay.

After extensive preparations, the Langsdorff Expedition departed with 40 people and 7 boats from Porto Feliz, by the Tietê river on 22 June 1826 and reached Cuiabá, in Mato Grosso on 30 January 1827.

[5] Due to the travel's hardships, Langsdorff team was unable to collect many biological specimens or study them in detail, so most of their account is geographic and ethnographic, being particularly interesting on the many indigenous people of Brazil they met, many of which became extinct.