Although a rapid response habituation to repeated stress exposure is a key characteristic of the hypothalamus-pituitary-adrenal (HPA) axis, several studies document a substantial inter-individual variability of such HPA response patterns. In order to further investigate the individual differences in the habituation of this important neuroendocrine system to psychosocial stress, 54 male twin pairs were exposed to moderate psychosocial stress on three occasions, each exposure separated by a 1-week interval. Additionally, an ACTH(1-24) stimulation test (1 microg) and a dexamethasone suppression test (0.5mg) were performed. Although on average the expected decrease of mean cortisol and ACTH responses across stress exposures was observed, only 52% of the subjects showed this well-documented general decline and almost 16% of the participants even showed a response sensitization across sessions. Furthermore, a weak habituation was related to low cortisol responses to both the first stress exposure as well as the ACTH challenge. Moreover, genetic analyses did not reveal any evidence for a substantial heritability of the individual cortisol response habituation or an association between this habituation and two common polymorphisms in the glucocorticoid receptor gene.