Although a variety of physiological and psychological stressors stimulate a significant increase in adrenomedullin (ADM) levels, suggesting a regulatory or protective role for ADM in countering the hypothalamo-pituitary-adrenal (HPA) axis activation following these stressors, it is still unknown whether light or darkness stress is involved in the endogenous ADM production systems. This study is aimed to investigate the effects of constant light or darkness for 60 days on ADM level in the plasma of adult male rats. ADM concentrations were assessed before and after the stressors in tail arterial blood by using HPLC. In the both groups, ADM levels greatly increased in the first week and than continued with lesser changes from the control levels. In conclusion, the study showed that keeping the rats in constant darkness and light vicinity for a long time altered ADM synthesis and secretion from the plasma or other tissues. (C) 2008 Elsevier B.V. All rights reserved.