ENA lot has been published about borrowings – especially at the appellative level – and the problems of their study in Baltic languages, however, only some publications are known on mutual borrowings at the level of onyms, resp., toponyms and anthroponyms, the data are not mapped and widely available to the public. Summarizing analysis is made difficult by the fact that the material to be covered is very broad and often hypothetically interpretable. The article briefly touches on theoretical questions about the potential phonetic, morphological, lexicalsemantic criteria identifying Lithuanianisms / Latvianisms or Letticisms. The analysis shows that there are significantly more place names of Lithuanian origin in the territory of Latvia than appellatives of Lithuanian origin in Latvian language. In the territory of Latvia, mainly in the border zone, there are mainly oikonyms, microtoponyms, and a few hydronyms; it is impossible to mention exact number (>600). Many toponyms are relatively new, coined from Lithuanian personal names. The reliability of the acquisition is strengthened by the parallels of the respective roots in Lithuania, as well as the localization of the place name. It cannot be denied that some of such Lithuanianisms could be called relative Lithuanisms, because they are made from appellatives borrowed earlier. Potential Letticisms (perhaps Curonianisms or Selonianisms) can be found mainly in northern Lithuania, they have also partly entered through anthroponyms. However, there are no summary studies on Letticisms in Lithuanian toponymy yet.There are about 140 given-names of Lithuanian origin (several of them are rare or arguable) in the “Dictionary of Latvian personal names” (Siliņš, 1990). About 30 of them are included in the Latvian modern calendar. On the other hand, only a couple of possible Letticisms were found in the dictionary of Lithuanian personal names (Kuzavinis & Savukynas, 1987) which are not used for naming newborns anymore. The number of mutually borrowed surnames is not large, studies on them are fragmentary. It is customary to consider surnames with the suffixes -aitis, -ūns / -uns, -utis as Lithuanianisms in Latvian anthroponymic system. Surnames with suffixes -niek- and often -in- in Lithuania indicate about possible borrowings, however Letticisms in the Lithuanian surname system should still be studied in depth. Keywords: Baltic languages, onymes, toponyms, anthroponyms, mutual borrowings. [From the publication]