The pharmacokinetics of 2-mercaptopropionylglycine (2-MPG) was studied in ten healthy volunteers after a single i.v. injection of 250 mg (1532 mumol). The total and non-protein-bound concentrations versus time curves were best described by a three-exponential function with terminal half-lives of 55 and 59 h respectively. Body clearance based upon the total concentration was estimated to be 105 and 231 ml/min based on the non-protein-bound 2-MPG. The corresponding values for Vss were 99 l and Vss,n 173 l, and for V gamma 485 l and V gamma,n 1121 l respectively. 75% of the dose was excreted in the urine, mainly during the first 6 h after injection. The proportion of non-protein-bound 2-MPG diminished exponentially during the first 15 h and then levelled off at about 30%. There was a nonlinear increase in the non-protein-bound fraction of 2-MPG as the total plasma concentration of the drug increased.