A revision of gomphodont cynodonts from the Chañares Formation of Argentina, is presented. Four species in two genera: Massetognathus pascuali, M. teruggii, M. major, and Megagomphodon oligodens were originally described by Romer in 1967 and 1972. Qualitative characters proposed originally as diagnostic of the species were revised, and a bivariate analysis of allometry of 17 linear skull measurements was performed upon 31 specimens. Allometry relationships account for most of the differences among specimens. There are neither size gaps nor systematic trends across the sample. The temporal region scales with a ′positive′ allometry, whereas components of the neurocranium and muzzle show ′negative′ or isometric scaling. This results in a change from short, stocky skulls with large muzzles in small specimens, to slender skulls with small muzzles and large temporal fossae characteristic of large individuals. The reduction and ontogenetic loss of the parietal foramen in a cynodont is reported for the first time. Consideration of qualitative characters together with the allometric results lead us to conclude that all forms belong to a single species in a growth series. Consequently, Massetognathus pascuali is recognized as the only valid entity, an idea previously suggested but without a justification based upon extensive analysis.