ENIn this paper, we present the 87Sr/86Sr data of 13 samples from horses from six Lithuanian burial sites dating from the 3rd to the 7th C AD. Alongside these data, we also publish the bioavailable 87Sr/86Sr data of 15 Lithuanian archaeological sites, based on 41 animals which enabled the construction of a reliable baseline for the Southeast Baltic area. The 87Sr/86Sr values partially confirmed the hypothesis that the unusually large horses found in Late Roman Period to Post-Migration Period burials are of non-local origin. Of the three non-local horses identified, two were among the largest specimens. However, the overlap of bioavailable 87Sr/86Sr data across different European regions does not permit us to establish whether the non-local horses originated from other areas in Lithuania or from more distant regions. With regards to the 87Sr/86Sr data, the place of origin of the non-local horses could be Southern Sweden. This encourages discussions on the possible directions of migration and compels us to rethink the current models that posit South and Central Europe as the main sources of migration. The results of the 87Sr/86Sr, δ13C, and δ15N analyses demonstrate that horses buried in the same cemetery had different mobility and feeding patterns. Differences could be due to the different function and sex of the horses as well as the lifestyle of their owners. The most sedentary horses were pregnant mares, while the extremely high δ15N of three horses may reflect additional fodder and probably a better diet. Keywords: 87Sr/86Sr analysis; horses; migration; mobility; Roman and Migration Period; Southeast Baltic. [From the publication]