A system for external quality assessment of serum drug determinations in the five Nordic countries has been established, and results of several surveys of antiepileptic drug determinations are described. The work was started as a pioneer work of NORDKEM at the time when NORDKEM was founded and was carried out during the years 1977-1979. The problems concerning methodology of drug analyses and, consequently, the problem of setting up quality assessment systems differ clearly from those in connection with the common analyses of clinical chemistry. Especially the quality of control materials is very important. An expert panel of drug specialists is necessary to plan and interpret an external quality assessment program of drug analyses, but it can very well be carried out in connection with other quality assessment programs. The state of the art of antiepileptic drug determinations in the Nordic countries at the time of the surveys was very good. The level of inaccuracy was well below 5%, and the level of imprecision was mainly in the range of 15-20%, although no outliers were excluded during the statistical treatment of the data.