Literary and linguistic activities
1774 - 1782: 3 volumes of the "Topographische Nachrichten von Lief- und Ehstland" (Topographical news on the Livonian and Estonian lands).The second volume focused on to the customs and traditions of the Latvian and Estonian folk, including music: Hupel described the polyphonic singing, the role of improvisation in folk music, and some musical instruments such as kokle, bagpipes, etc. He prepared the notations of two Latvian folk tunes; one of them is considered to be the oldest published sun-worshiping song tune;
1780: "Ehstnische Sprachlehre für beide Hauptdialekte, den revalschen und den dörptschen; nebst einem vollständingen Wörterbuch" (Estonian language teaching aid for the two main dialects, Revel and Tallinn; complemented by a dictionary). It was printed twice - in Riga in 1780 and in Jelgava in 1818;
1781 - 1798: prepares for publication a series of scholarly and popular scientific articles on the Baltic, its history, geography, anthropology, culture "Nordische Miscellaneen" (1781-1791) - 28 volumes and "Neue Nordische Miscellaneen" (1792-1798) in 18 volumes is follow-up edition.
Folkloristics
1777: Hupel is one of those, who, in response to the German philosopher J.G.Herder's call, organized Latvian folk songs collection, mainly involving local clergy. Having received the texts, he sent them to J.G. Herder to Germany, together with his own collection of Estonian songs, and translated the texts into German. (The collection and description of the folk songs sent to J.G. Herder can be found here.)