Following administration of 0.6 mg kg−1 rocuronium, the pharmacokinetics and the pharmacodynamics were studied in six obese and six control (normal weight) patients receiving balanced anaesthesia. Twelve gynaecological patients were allocated into two groups, according to body mass index (normal weight: body mass index: 20–24, obese weight: body mass index > 28). Venous plasma concentrations were determined by high-pressure liquid chromatography before administration of rocuronium, at 1, 2, 4, 6, 8,10, 15, 20, 25, 30, 35, 40, 48, 60, 75, 120, 180, 240, 300, 360 and 420 min after administration of rocuronium and at recovery of single twitch to 25% and 75% of control twitch height. Onset time was shorter (NS) in the obese compared with normal weight (obese weight: 65 ± 16, normal weight: 100 ± 39 s, mean±SD). Duration 25% (obese weight: 29.5 ± 5.3, normal weight: 28.4±5.3 min) and spontaneous recovery time (obese weight: 12.6 ± 2.7, normal weight: 12.5 ± 2.3 min) did not show any differences between the two groups. The pharmacokinetics of rocuronium were comparable in the two groups. The volume of distribution at steady state Vss (mL kg−1) was 208 ± 56 in normal weight and 169 ± 37 in obese weight. Distribution (T1/2α) and elimination half-life (T1/2β) as well as mean residence time were 15.6 ± 3.7, 70.3 ± 23.9 and 53.2 ± 9.8 min in normal weight and 16.9 ± 3.8, 75.5 ± 25.5 and 51.1 ± 18.9 min in obese weight,respectively. Also, no differences were observed in plasma clearance (3.89 ± 0.58 in normal weight and 3.62 ± 1.42 mL kg−1 min−1 obese weight). This study indicates that the pharmoacodynamics and pharmacokinetics of rocuronium are in female patients not altered by obesity.