The salivary cortisol response to an acute psychological stress challenge was investigated in normal male undergraduate students. A modified version of the Trier Social Stress Test (TSST) was used and saliva collected on 6 occasions before during and after the stress challenge. Control subjects were allowed to read quietly. As expected the cortisol response in experimental subjects was robust and peaked 12 minutes after the end of the stress. Endogenous monoamine oxidase A inhibitory activity (MAO-AI) was measured in the same saliva samples. MAO-AI also changed in response to the stress challenge, peaking in the saliva sample collected immediately after the stress challenge, 12 minutes prior to the cortisol peak sample. Furthermore the degree of increase in salivary MAO-AI was found to predict the degree of cortisol increase in the test subjects (r=0.76; n=14; p<0.001). These results are consistent with the hypothesis that elevated central monoamines, driven by inhibition of their main metabolic enzyme, can activate the hypothalamic-pituitary-adrenal (HPA) axis in the stress response. This finding lends further support to the notion that endogenous generation of MAO-AI is a normal homeostatic regulatory mechanism.