The present study investigated cortisol responses to three different stimulation procedures, with a focus on the contribution of genetic factors. Thirteen monozygotic (MZ) twin pairs and 11 dizygotic (DZ) twin pairs performed bicycle ergometry until exhaustion and were exposed to the psychological stress of public speaking and mental arithmetic in front of an audience. Furthermore, 9 MZ pairs and 10 DZ pairs were injected with 100 micrograms synthetic human CRH (hCRH). The adrenocortical response to these challenges was monitored by determination of cortisol in saliva. Significant intraindividual stability of baseline cortisol levels was found in females, but was less in males. Maximum cortisol responses to all three stimulation procedures were significantly intercorrelated in males, but in females only the cortisol responses to hCRH and ergometer exercise showed a significant correlation. While a decided influence of genetic factors was observed for all three baseline cortisol levels as well as for the response to hCRH, heredity appeared to be play a minor role in the adrenocortical response to psychological stress. Cortisol changes after bicycle ergometry revealed no impact of genetic factors on the secretion of cortisol in response to strenuous physical exercise.