We study some topics around Łoś’s theorem without assuming the Axiom of Choice. We prove that Łoś’s fundamental theorem on ultraproducts is equivalent to a weak form that every ultrapower is elementarily equivalent to its source structure. On the other hand, it is consistent that there is a structure M and an ultrafilter U such that the ultrapower of M by U is elementarily equivalent to M, but the fundamental theorem for the ultrapower of M by U fails. We also show that weak fragments of the Axiom of Choice, such as the Countable Choice, do not follow from Łoś’s theorem, even assuming the existence of non-principal ultrafilters.
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