Although the effects of melatonin on vascular smooth muscle and vascular reactivity are described in last decade, the effects of melatonin on vascular tissues are still vague. The current study investigated the effects of pinealectomy on the blood vessel wall in the chicken. Fifteen chicks were divided into three groups, namely unoperated control, sham-operated and pinealectomized chicks. At the end of the experiment, a quantitative structural analysis of 15 sciatic artery samples from these groups was investigated. Effects of pinealectomy were quantified using volume fraction approach of stereological method. In this study, total cross-sectional area of the artery in pinealectomy group was significantly higher than those in unoperated control and sham-operated animals (p < 0.05). However, surgical pinealectomy procedure resulted in a significantly decreased thickness of the tunica adventitia of the vessel wall, in contrast to those of tunica media (p < 0.01). The present results indicate that pinealectomy had a vasodilator effect on the sciatic artery. In the light of current study, the structural changes in the vessel demonstrated by quantitative morphometric methods have been interpreted as a reflection of the role of melatonin on vessel wall and vascular reactivity, but this suggestion need to be validated in the human setting. © 2009 OMU AU rights reserved.