Background and objective: Arterial grafts are prone to vasospasm. Opioid analgesics are commonly used in the perioperative course of cardiac surgical procedures. Therefore, we investigated the direct effects of morphine, meperidine, fentanyl and remifentanil on the human radial artery. Methods: Radial artery segments, obtained from 20 patients, were precontracted with phenylephrine. Using the organ bath technique, the endothelium-independent vasodilatation was tested in vitro by addition of cumulative concentrations of morphine, meperidine, fentanyl and remifentanil in separate organ baths, in the presence or absence of naloxone. Indomethacin and NG-nitro-l-arginine methyl ester was added to all organ bath in order to determine the effects of prostaglandins and nitric oxide, respectively. Results: Morphine (10−8–10−4mol L−1), meperidine (10−10–10−6mol L−1), fentanyl (10−10–10−6mol L−1) and remifentanil (10−8–10−4mol L−1) caused a concentration-dependent vasorelaxation in the human being artery rings. The relaxations in the presence of naloxane did not change. The maximal relaxant effects of meperidine and fentanyl were significantly greater than those of morphine and remifentanil (P < 0.05). Conclusions: These findings indicate that morphine, meperidine, fentanyl and remifentanil produce concentration-dependent and endothelium-independent relaxations in human being radial artery rings. Meperidine and fentanyl are more potent relaxant agents than morphine and remifentanil in the human being radial artery in vitro.