The accuracy of the fluorescent (FM) and radioactive microsphere (RM) techniques is similar in acute experiments but has not been established in chronic experiments. In the present study various combinations (at least pairs) of FM and/or RM labels were injected simultaneously between 2 mo and 5 min before each animal was killed. Blood flow was determined in many organs. Intramethod mean difference and variation did not change over time for FM but increased significantly for RM (from 1.8 +/- 1.4 to 25.6 +/- 21.8% and from 4.4 +/- 3.2 to 32.4 +/- 23.0% at 5 min and 2 mo, respectively). Also the FM-RM intermethod mean difference and variation increased (from -0.5 +/- 8.5 to 40.8 +/- 23.8% and from 23.6 +/- 4.6 to 71.8 +/- 34.3%, respectively). After 2 mo, blood flow estimations were 20-50% lower with the various RM, whereas brain and liver blood flow values varied even more between isotopes. Underestimation started within 1 day for Cr-51 and within 2 wk for Ce-141, Nb-95, and Sr-85. We conclude that FM are superior to RM for blood flow determination in experiments lasting longer than 1 day, presumably because of leaching of isotopes from RM.