ABSTRACT Neolicaphrium recens (Mammalia, Litopterna) was a slender South American Native Ungulate (SANU). The assessment of its body size is a big challenge because there are no living representatives and no clear close morphological analogues. In this contribution, we report for the first time a femur of N. recens exhumed from Late Pleistocene sediments of Uruguay, improving the knowledge of the postcranial skeleton of the species. This stylopodial element is important for doing palaeobiological inferences, including estimating the body mass. We performed body mass estimations by previous allometric equations. As a method to directly test the models and their usefulness in fossils, we applied the equations on six specimens of extant deers of known body weight and with skeletons preserved. According to the obtained data, the transverse width at the middle of the diaphysis and the circumference of the bone at that level can be used with confidence to estimate the body mass of fossil ungulates. We point out a set of more probable values of body mass for N. recens: 28.76298 ± 10.6412, 27.78496 ± 8.0576, and 26.72 ± 10.42 kg. Morphological variables as well as the models for estimating body masses in fossils are widely discussed.