Though the development of novelistic prose in medieval Castilian literature has been traditionally associated with the encyclopedic works of Alfonso el Sabio and those of his nephew, Juan Manuel, author of the Conde Lucanor, the stylistic differences between these two writers are sufficiently important to indicate the existence of a transitional period characterized by the absorption of classical rhetoric into the mainstream of religiously oriented didactic literature. An analysis of the critically neglected works of Sancho IV, second son of Alfonso and guardian to the young Juan Manuel, reveals the intermediate stages in the transformation of impersonal Alfonsine historical narrative to the highly anecdotal mode of Juan Manuel and subsequently affirms Sancho's role in Castilian letters while stressing Juan Manuel's clear genius for synthesizing the major literary currents of his age. (In Spanish)