To reveal the mechanisms of adaptation to occupational factors, the authors screened the workers exposed to noise for changes of pulse rate, blood pressure, catecholamines content, auditory threshold within a day. The study showed that adaptation to unfavorable work conditions deteriorates within a day (from morning to night shifts). Reliability of catecholamines level was conditioned by the night fall of catecholamines level and its changes within a day. Compromised adaptation to chronic unfavorable occupational factors result in unstable catecholamines level. Functional state of hearing is mediated by vegetative state including catecholamines content.