A chemical-shift selective (CHESS) 19F MR imaging technique was used to map selectively the antineoplastic drug 5-fluorouracil (5-FU) and its major catabolite alpha-fluoro-beta-alanine (FBAL) in tumor-bearing rats. The pulse sequence employed a CHESS RF saturation pulse to suppress either the 5-FU or the FBAL resonance before the other component in the two-line 19F MR spectra was measured. Selective 5-FU and FBAL images with a spatial resolution of 10 x 10 x 15 mm3 (1.5 ml) were obtained in 40 min from six ACl rats with implanted Morris hepatoma. Because the transmitter frequency could always be set to the Larmor frequency of the 19F resonance employed for imaging, the images were free of chemical-shift artifacts in readout and slice-selection direction. Whereas FBAL appeared only in the liver, the kidneys, and the bladder, 5-FU could also be detected in all major organs and in the muscular system. In the Morris hepatomas, a small 5-FU uptake and no FBAL accumulation were measured. The CHESS 19F MRI technique provides useful physiological and biochemical data on the biodistribution of the antineoplastic drug 5-FU and on the different catabolic activities of the tissues.