The endocasts of the cynodonts Massetognathus sp. and cf. Probelesodon are studied from descriptive and quantitative viewpoints. The morphology of the casts is described briefly, doing special attention to those features that import to the quantitative analysis, as for instance the general appearance of the casts and some vascular impressions. The general conclusions of this study are: 1.--Massetognathus sp. has had a brain with rather long olfactory peduncles and perhaps not greatly developed olfactory bulbs which could imply a non-macroosmatic condition, up to date not known for cynodonts. Cf. Probelesodon, on the other hand, shows a brain type similar to those of other cynodonts. 2.--Certain cynodonts at least, have had brains that filled fairly well the endocranial cavity. 3.--The cynodonts Massentognathus sp. and cf. Probelesodon, as surely other species, have acquired in Middle Triassic times a relative brain size rather closed to that of certain fossil and living mammals.