The article is devoted to the aspectual semantics of simplex (non-prefixed) verbs in Old Russian. Traditionally, these verbs have been considered to be aspectually unspecified in the Old Russian period. However, our analysis shows that already in the ancient period the class of simplexes was heterogeneous with respect to aspectual semantics; several subclasses can be distinguished. The article offers a method (based on a complex approach that takes into account a number of morphological and semantic parameters) and algorithms that allow us determine the aspectual meaning of a particular verbal lexeme. It is also argued that for determining the aspectual status of a perfective or an imperfective verb morphological and semantic parameters (and prohibitions) have different degrees of importance. That indicates a certain asymmetry of the aspectual system observed in the Old Russian language.