Several approaches for calibrating microdialysis probes for exogenous compounds in vivo are described which avoid the error introduced by in vitro calibration. These methods are based on establishing a steady state of the exogenous compound by a continuous (zero-order) iv infusion. The steady-state concentration is estimated by three methods that directly determine the in vivo concentration. The methods are (a) extrapolation of dialysate concentrations at various flow rates to the concentration at zero flow, (b) dialysis with concentrations of analyte added to the perfusion medium above and below the expected concentration to determine the concentration at no net flux across the membrane, and (c) dialysis at a very slow perfusion rate (57 nL/min) where the recovery is expected to be better than 90%. Using these approaches, the recovery for cocaine in the brain was found to be (8.9 +/- 0.68)%, as compared to an in vitro recovery of (5.1 +/- 0.18)% at 24 degrees C and (7.4 +/- 0.18)% at 37 degrees C, at a perfusion rate of 1.2 microL/min through a 0.3- X 2-mm microdialysis probe. The in vivo concentration of cocaine in the rat brain for an intravenous dose of 0.3 mg/kg per min was found to be 17.1 +/- 1.3 microM.