ENThe academic and scientific activities of Latvian linguist Jānis Endzelīns cannot be fathomed without knowledge of Lithuania and the Lithuanian language. Endzelīns’s activities were enriched by both interest in the country and contacts with Lithuanian contemporaries. It is often mentioned that Endzelīns established close relations with Kazimieras Būga (1879–1924). Būga’s letters to Endzelīns are stored in the library of Vilnius University and the Institute of Lithuanian Literature and Folklore; some of the letters have been published (approximately 60 letters and postcards). The linguists actively corresponded on issues of Baltic linguistics: they discussed dialects, toponyms, dictionary matters, et cetera. They also discussed more personal information reflecting known biographical facts. Endzelīns had many distinguished colleagues and students, and their mutual influences in their professional lives were noticeable. Examples include Lithuanian professor Jurģis Ģerulis, Lithuanian students who studied Baltistica at the University of Latvia, and Petrs Joniks, who raised his classification at the University of Latvia to obtain a professorship at the Vytautas Magnus University. In 1939–1941, Jonas Kabelka attended Endzelīns’s lectures at the University of Latvia; he published memoirs both about the lectures and the professor himself. In 1953, Algirdas Sabaliauskas and Jonas Kazlauskas participated at a student scientific conference in Riga and visited Endzelīns, as is recorded in the memoirs of A. Sabaliauskas.Lithuanian linguists congratulated the professor of Baltic Studies on the occasions of his life and work anniversaries and also participated in his funeral (speeches were delivered by Kabelka and Sabaliauskas). Photographs from anniversaries and funerals are stored in the archive of the UL Academic Library, the Latvian National Library, and the Vilnius University Library. Keywords: history of linguistics, Jānis Endzelīns, Lithuanian linguists, Kazimieras Būga, Jonas Kabelka, Algirdas Sabaliauskas.