The aim of the present study was to investigate the effect of intravenously (i.v.) administered fentanyl and clonidine on ventilation in 12 healthy male volunteers (age 30.8±4.9 years) who either received fentanyl alone (1.5≥μg kg−1) or fentanyl (1.5≥μg kg−1) in combination with clonidine (3≥μg kg−1). The effect on ventilation was measured with a CO2 rebreathing system. The respiratory depression caused by fentanyl disappeared 120 min after injection. The corresponding slopes were 7430±2075 mL kPa−1 prior to (t0) and 6263±1864 mL kPa−1 120 min post-application (base-line vs. t120; P=0.106). An impaired ventilatory response was observed during CO2 rebreathing at t120 after the injection of fentanyl and clonidine. Before drug administration, the slope of the response curves was 7700±2800 mL kPa−1, which was reduced to 5480±2135 mL kPa−1 (P<0.035) at t120. These data suggest a prolongation of a fentanyl-induced ventilatory depression when used in combination with clonidine.