Fazadinium was administered as an intravenous bolus of 0.5 or 1 mg/kg to nine patients during neurolept-anaesthesia. Continuous recording of neuromuscular activity and serial collection of blood samples for drug assay allowed the relationship between neuromuscular blockade and plasma fazadinium concentration to be studied. Pharmacokinetic analysis demonstrated high plasma clearances (221 and 185 ml/min), and short terminal half-lives, (30 and 50 min), for the two doses of the drug. Mean plasma concentrations at 10 and 50% recovery were 1.42 and 0.88 micrograms/ml respectively, demonstrating that the potency of fazadinium is approximately half that of tubocurarine and one eighth that of pancuronium. In addition, the relationship between plasma concentration and depression of neuromuscular activity during spontaneous recovery from the relaxant is described.