The extent of embolization of different sized radioactive microspheres in experimental tumours and the homogeneity of their distribution in normal liver was examined in 25 rats. The ratio of arterially introduced microspheres lodging in tumour tissue compared to the surrounding normal hepatic parenchyma was measured for 15, 32.5 and 50 microns diameter tracer microspheres. The mean tumour to liver arterial perfusion ratio (T/L) for 15 and 32.5 microns spheres was approximately 3:1 in both cases and there was no significant difference between these values (P greater than 0.05). However, 50 microns microspheres did not preferentially lodge in malignant tissue (mean T/L ratio 1:1). The homogeneity of distribution of microspheres embolizing in the normal liver tissue was assessed for each microsphere size. As microsphere diameter increased from 15 to 50 microns, microspheres lodged more evenly throughout the liver substance. For 15 microns microspheres the coefficient of variation was 55.5% +/- 8.3 and 32.5 microns microspheres distributed with a coefficient of 35% +/- 16.8 while 50 microns spheres distributed most evenly with a coefficient of 19.7% +/- 6.8.