Batch-wise pH-adjustment of plasma samples for free concentration determinations can be performed directly in the ultrafiltration devices using a CO(2) incubator. The pH of the samples could be adjusted to within a sufficiently narrow interval though the initial differences in pH were large. The possibilities for simultaneous determination of free and total concentrations were studied using NAD-299 as a model compound. By adding a known concentration of a (13)C-labelled isotope of the studied drug to the sample prior to ultrafiltration it was possible to calculate the total concentration from the ratio of the drug peak area to the isotope peak area while the free concentration was calculated from the drug peak area. Initial experiments showed good precision and accuracy as well as a good correlation with concentration data acquired in the conventional way.