BACKGROUND: Dosage guidelines for muscle relaxants are based on dose-response studies, normally performed after several minutes of stable nitrous oxide (N O)-opioid anesthesia. However, relaxants are used immediately after induction of anesthesia. The study was designed to determine the influence of the duration of anesthesia and N O on the onset time at the adductor pollicis (AP) and the corrugator supercilii (CS) muscles of maximum neuromuscular blockade after mivacurium. METHODS: After institutional approval and informed consent, patients were randomly allocated into three groups. Anesthesia was induced with alfentanil and propofol. Group A (n = 10) received mivacurium (0.1 mg/kg) immediately after loss of consciousness. Groups B (n = 10) and C (n = 10) received mivacurium after 15 min of anesthesia with propofol alone (B) or propofol with N O (C). The evoked response to train-of-four stimulation was measured by acceleromyography at the AP and the CS. RESULTS: Maximum neuromuscular blockade (%T1, median [range]) was significantly less in group A than in groups B and C ( < 0.001) at both the AP (81 [47-90]; 90 [35-100]; 100 [93-100], respectively) and the CS (19 [5-63]; 68 [61-100]; 89 [72-100], respectively). Maximum neuromuscular blockade was less in group B than in group C ( < 0.001) at the AP. Onset time of maximum neuromuscular blockade was not different between groups but was shorter at the CS than at the AP. CONCLUSIONS: Duration of anesthesia and N O before mivacurium injection affect intensity of neuromuscular blockade but not onset time. Neuromuscular blockade obtained at the AP after several minutes of stable anesthesia with N O is greater than immediately after induction. This explains in part the discrepancy between the measured ED and the intubating dose.