Abstract Introduction. Melatonin, the major pineal secretion, regulates sleep/wake cycle and plays an important part in several physiological functions. It is a powerful free radical scavenger and upregulates several antioxidant enzymes to reduce oxidative stress. Objective. Mammalian red blood cells contain membrane-bound acetylcholin- esterase. Melatonin, being an effective antioxidant, may cause alteration in erythrocyte acetylcholinesterase activity. Methods. The study was carried out on 29 healthy donors of both sexes who gave informed consent for the use of their blood samples for the study. The erythrocytes isolated from blood collected at two different timings of the day were subjected to in vitro treatment with melatonin in a concentration- dependent manner followed by the assay of enzyme activity. Results. A rhythmic pattern is observed in the activity of acetylcholinesterase, the activity being higher in diurnal samples. Exogenous melatonin modulates the activity of acetylcholinesterase in erythrocytes which is more pronounced at such concentration close to the peak physiological level of melatonin in the body. Conclusion. The increase in the enzyme activity may be explained in relation to the antioxidant potential of melatonin, which is in accordance with the change in photic signals. Further study is required to understand the mechanism of action. Key words: acetylcholinesterase, melatonin, oxidative stress, erythrocytes. INTRODUCTION Melatonin (N-acetyl-5-methoxy- tryptamine) is involved in the regulation of circadian rhythm. The endogenous pineal hormone is a derivative of an essential amino acid L-tryptophan and is involved in several physiological functions such as cardioprotection, immuno-stimulation, thermoregulation, sleep-induction and geronto-protection (1). Melatonin acts as a potent antioxidant and a powerful free radical scavenger (2). It has also been reported to maintain the level of glutathione and of several antioxidant enzymes. The indoleamine provides protection against neurodegeneration, and is active in tumour-suppression and oncostasis. Melatonin’s amphiphilic nature enables 311 *Correspondence to: Rizvi Syed Ibrahim, University of Allahabad, Faculty of Science, Department of Biochemistry, Allahabad, 11002, India. Email: sirizvi@gmail.com Acta Endocrinologica (Buc), vol. VII, no. 3, p. 311-316, 2011 MODULATION OF ACETYLCHOLINESTERASE ACTIVITY BY MELATONIN IN RED BLOOD CELLS S.I. Rizvi* and S. Chakravarty University of Allahabad - Department of Biochemistry, Allahabad, India General Endocrinology doi: 10.4183/aeb.2011.311