Two groups of preschoolers and one group of young grade-schoolers were tested for their comprehension of presuppositions and negation in complex syntax. Four types of sentences were presented to each child: affirmative and negative versions of sentences with factive main predicates, which presuppose the truth of the proposition of the complement clause, and nonfactive main predicates, which do not. A forced-choice design was used: the children chose the agent subject mentioned in the complement clause, thus affirming the complement; or the unmentioned agent, thus denying the complement. Five factive predicates and five non-factive predicates were used so as to permit a comparison within each group. Competence increased into the early school years: the oldest group of participants showed a fair mastery of the syntax-semantics of the predicates of the study. The younger children showed errors of two different kinds, described as the Overextended Negation Tendency and the Overextended Affirmation Tendency. Both of these errors decreased markedly in the oldest group. The non-unitary nature of the acquired competence is discussed. In particular, it is pointed out that (1) factivity is not a grammatically marked operation and as such leads to what appears to be a gradual acquisition pattern; (2) the test of factivity comprehension employed here demanded a competence beyond that of normal use.