The purpose of this study was to compare the effects of fentanyl and remifentanil on splanchnic perfusion during coronary artery bypass graft (CABG) surgery. Fifty patients were randomized to receive either fentanyl (10 mu g.kg(-1) at induction and 5 mu g.kg(-1).h(-1) infusion for maintenance) or remifentanil (3 mu g.kg(-1) at induction and 1 mu g.kg(-1).min(-1) infusion for maintenance). Patients in both groups were comparable with regard to demographics. Intraoperative volume management and inotropic therapy were similiar in both groups. Regarding heart rate, there were no significant differences between the groups at any measurement time (P > 0.05). Compared to the fentanyl group, the remifentanil group showed a significant decrease in mean arterial pressure during induction. Also, the gastric intramucosal CO, pressure (Pg(CO2)) and the Pg(CO2) gap, defined as the difference between Pg(CO2) and Pg(CO2) were significantly increased and the gastric mucosal pH (pHi) was significantly decreased in the remifentanil group in the postinduction period (P < 0.05). However, there were no statistically significant differences in respiratory data at any time between the two groups (P > 0.05). Both fentanyl and remifentanil seemed to be effective and well tolerated in this CABG population. Episodes of hypotension and transient reduction in splanchnic perfusion were more common in patients treated with remifentanil when compared to those receiving the fentanyl opioid regimen.