BACKGROUND: Rocuronium is a non-depolarising neuromuscular blocking agent structurally related to vecuronium. The compound has a rapid onset and an intermediate duration of action. The rapid onset is of importance in patients at risk for pulmonary aspiration, for elective induction of anaesthesia slower onset properties generally are accepted. In this context, we asked whether the induction dose of rocuronium may be reduced to doses smaller than 2 x ED95 in situations in which slower onset properties may be acceptable. METHODS: The time course of neuromuscular block and intubating conditions of two different doses rocuronium, 2 x ED95 (0.6 mg/kg) and 1.3 x ED95 (0.4 mg/kg), were investigated in 90 patients. We first determined the time course of neuromuscular block using electromyography (EMG), n = 15 for each group. In the second part the intubating conditions 3 min after injection of either rocuronium 0.6 mg/kg or rocuronium 0.4 mg/kg were evaluated, n = 30 for each group. RESULTS: In the present study reduction of the dose of rocuronium led to a slower onset (148 +/- 32 s vs. 220 +/- 30 s; P < 0.05) and a shorter clinical duration (21 min +/- 4 vs. 36 +/- 7 min; P < 0.05). The recovery index was modified by the dose reduction: 11 +/- 3 min after 0.6 mg/kg rocuronium and 9 +/- 2 min after 0.4 mg/kg. After both doses of rocuronium the intubating conditions were good to excellent, no difference between both rocuronium groups were found. CONCLUSION: In the present study dose reduction from 0.6 mg/kg rocuronium to 0.4 mg/kg rocuronium led to a slower onset and reduced clinical duration. However, the intubating conditions, evaluated 3 min after injection of the muscle relaxant were comparable. This offers new possibilities for muscle relaxation for surgical or diagnostic procedures of short duration and may reduce costs.