The sequential hypothesis about depression tells: "neurotic" and "endogenous" depression are not different diseases, but two different moments in the evolution of a unique disorder (initial and terminal, respectively). The authors use Kiloh and Garside's (1963) and Carney et al's (1965) data to support this hypothesis. The bibliographic data are analyzed in that way to show that biological correlates of endogeneity are the final pathway of an ontogenic evolution of biological modifications in depression less obvious in initial episodes ("neurotiforms") and more evident in subsequent episodes ("endogeniforms").