SIZE AND BODY MASS IN PROCARIAMA SIMPLEX ROVERETO, 1914 (AVES: PHORUSRHACIDAE: PSILOPTERINAE). Body-mass of fossil birds can be estimated using comparisons with extant birds, taking into account that the measures of hindlimbs are indicators of body weight. Hindlimb bones (femur, tibiotarsus, and tarsometatarsus) would represent a direct relationship with body mass, because they provide, in different degrees, the support of the body. In this contribution, the body-mass of fossil specimens of Procariama simplex Rovereto from the Miocene of Argentina is calculated using two different estimates. The first model assumes that there is geometric similarity between fossil and analogous extant birds, in this case between Procariama simplex and its analogous Cariama cristata Linnaeus. The second model, more used in birds, considers the influence of the size or body-mass on the form and function of the skeleton. It considers the logarithmic relationship between body-mass and the minimum circumference of the shaft of the hindlimb elements involved in the support of a bird. Both models estimate values between 10 and 13 kg for Procariama simplex, higher than previous estimates. These values resemble in size (mass) those of a Patagonian rhea Pterocnemia pennata d'Orbigny. Although this Rheidae is not related phylogenetically to Phorusrhacidae, they share the same morphotype with long hindlimbs, suitable for running.