Summary During adolescence pubertal development is said to lead to an increase in general stress sensitivity which might create a vulnerability for the emergence of psychopathology during this period. However, the empirical evidence for increasing stress sensitivity is scarce and mixed. Biological responses (salivary cortisol and alpha-amylase) were investigated during a social-evaluative stressor, the Leiden Public Speaking Task, in 295 nine to 17-year olds. Specific attention was paid to different elements of the task, that is anticipation to and delivery of the speech. Biological reactivity to the speech task increased with age and puberty, particularly during anticipation. Current findings support the idea that biological stress sensitivity increases during adolescence, at least in response to a social-evaluative situation. The increasing stress sensitivity appears related to both age and pubertal maturation, but unique contribution could not be distinguished. The importance of measuring anticipation is discussed.