Salivary cortisol is frequently used as a biomarker of psychological stress. 
However, psychobiological mechanisms, which trigger the 
hypothalamus-pituitary-adrenal axis (HPAA) can only indirectly be assessed by 
salivary cortisol measures. The different instances that control HPAA reactivity 
(hippocampus, hypothalamus, pituitary, adrenals) and their respective 
modulators, receptors, or binding proteins, may all affect salivary cortisol 
measures. Thus, a linear relationship with measures of plasma ACTH and cortisol 
in blood or urine does not necessarily exist. This is particularly true under 
response conditions. The present paper addresses several psychological and 
biological variables, which may account for such dissociations, and aims to help 
researchers to rate the validity and psychobiological significance of salivary 
cortisol as an HPAA biomarker of stress in their experiments.