Mass balance pharmacokinetics, with simultaneous blood sampling from an artery and the internal jugular vein, was used to characterize the cerebral uptake of ketamine, norketamine, and midazolam in normoventilated pigs. Intravenous injections of ketamine or midazolam decreased the cerebral blood flow (CBF) by one third, as measured by intermittent 133Xe washout. By means of pharmacodynamic models, the effects on the CBF could be predicted from the arterial drug concentrations. The high-resolution CBF vs. time curves thus generated allowed the calculation of cerebral drug levels from arterio-venous concentration gradients in spite of a continuously changing regional blood flow. By their effects on the CBF, ketamine and midazolam decrease their own rate of transport to the brain, the immediate 30-35% drops in CBF giving similar reductions in initial net influx of drug. Physiological pharmacokinetic models assuming a constant regional blood flow are therefore not appropriate. Under clinical conditions, the CBF is determined mainly by the effects of the anesthetics and by the arterial CO2 tension. CBF changes in either direction influence the transport of drugs to the brain and may consequently result in impaired or exaggerated drug effects.