The effects of respiratory alkalosis and dehydration on the pharmacokinetics of sufentanil have been determined. Forty neurosurgical patients entered the study: group I (n = 20) had spinal surgery while group II (n = 20) had elective craniotomies. All patients received a single intravenous bolus of sufentanil (2 micrograms/kg) during induction of anesthesia. Patients in group II received furosemide (40 mg) and mannitol (1 g/kg) prior to surgery and were hyperventilated to a PaCO2 below 30 mmHg. The anesthetic management was identical in both groups. Arterial blood samples were collected to determine sufentanil concentrations (RIA), blood gases, pH, hematocrit and electrolytes. Patients in group II had significantly higher blood pH and urinary output when compared to group I (p less than 0.05). Plasma sufentanil concentrations were best fitted to a bi-exponential curve and the pharmacokinetic variables calculated using a curve stripping program (STRIPE). No significant differences were observed between groups regarding plasma sufentanil concentrations or pharmacokinetic parameters. Our results demonstrate that respiratory alkalosis and dehydration do not alter the pharmacokinetics of sufentanil after intravenous bolus administration.