Prior experiments have shown that sentences such as (1) Mary bakes bread but no cookies lead to a reduced accessibility of the concept mentioned in the negated phrase, whereas sentences such as (2) Elizabeth burns the letters but not the photographs do not. In the present article, two explanations for this result are investigated. According to situation model theory (Johnson-Laird, 1983; van Dijk & Kintsch, 1983), the reason is that the entity mentioned within the negated phrase in (2) is not absent from the described situation. According to discourse representation theory (Kamp, 1981), in contrast, the negation operator in (2) does not reduce the accessibility of the negated concept, because the corresponding discourse referent is not introduced but merely referred to within the operator's scope. In two experiments, participants were presented with narrative texts including negation sentences that either introduced or referred to entities, and that either described a situation in which only the nonnegated or only the negated entity was present. The accessibility of the relevant concepts was measured by means of a probe recognition task. The results support the situation models explanation.