A number of investigators are using the quantitative no-net-flux microdialysis technique to monitor basal neurotransmitter dynamics in discrete brain regions of behaving animals. The predictive validity of the probe extraction fraction (Ed) for quantifying decreases in the rate of dopamine (DA) clearance from the extracellular space is well documented. It was recently suggested, however, that Ed may be insensitive to increases in DA clearance. Here we report that the Ed for DA in the nucleus accumbens (NAc) of the behaving mouse is increased following pharmacological inactivation of kappa-opioid receptors, a treatment previously shown to augment DA uptake. The Ed obtained in control mice and those that received the long-acting kappa-opioid receptor antagonist, nor-binaltorphimine (nor-BNI), satisfied the requirement that the mean values of each were lower than the mean value in vitro for the same probes immersed in well-stirred artificial cerebrospinal fluid. The Ed was increased in the NAc of nor-BNI-treated mice as compared to saline-treated control animals. The corresponding increase in the DA uptake rate was quantified by using the Ed values to calculate a change in the apparent clearance rate constant. Nor-BNI treatment did not alter the apparent extracellular dopamine concentration represented by the point of no-net-flux indicating that the rates of DA uptake and release were both increased.