Barn Owls (Tyto alba) have been considered a useful tool for estimating extinct and extant distributions of small mammals by the analysis of their diets. To test Barn Owlś' sensitivity to environmental changes, we analyzed the trophic ecology of these owls in northern Argentine Patagonia, a region characterized by a marked west-east vegetative gradient. We based our study on new and published information on diets in 15 localities along this gradient, from the Andes to the Atlantic Ocean. We analyzed number of mammalian prey items, food niche breadth, and mean weight of prey. We used Barn Owls' food habits to detect changes in the local composition of prey species, by means of correspondence and cluster analysis. Our results confirmed Barn Owls as small-mammal specialists (up to 99% of their total prey). The number of mammalian prey species and the mean weight of prey decreased from west to east, and food niche breadth was not correlated with longitude. Statistical analyses yielded an ordination of localities that corresponded to changes in vegetation and in small-mammal assemblages. Our results in northern Patagonia showed that prey selection along a vegetative gradient was associated with the rodent assemblages in each vegetation type. This suggests that the use of Barn Owl pellets is appropriate for study of the distribution of small mammals.