5-Fluorouracil (5-Fu) and 5-Fluorodeoxyuidine (FdUrd) are substances widely used in the chemotherapy of cancers. The exploration of the metabolic pathways, is of great interest when they are combined with drugs which are likely to alter their biological response. The method described consists in the detection with ion-pair microbore HPLC of minimum amounts, ranging from 100 to 500 pg. © 1990, Taylor & Francis Group LLC. All rights reserved.