The use of a new configuration to control the automated sequential trace enrichment of dialysate (ASTED) system has been used to estimate 1-(beta-D-arabinofuranosyl)-5-(1-propynyl)uracil and its metabolite 5-propynyluracil in urine. The system employs "heart-cutting" as a means to improve the efficiency of sample preparation and reduce analysis time. Using this technique the mean within- and between-run imprecision (coefficient of variation) at three different urine analyte concentrations was found to be 1.6 and 3.6% and 1.7 and 3.3% for 5-propynyluracil and 1-(beta-D-arabinofuranosyl)-5-(1-propynyl)uracil, respectively.