The use of [3H]diisopropylfluorophosphate [( 3H]-DFP) to measure erythrocyte survival is well documented; however, the procedure has not been widely used in animal research primarily because it is more labor intensive than other methods. The [3H]DFP method was modified to simplify the preparation of samples for scintillation counting by using whole blood and to allow for small-volume serial blood sampling of individual animals. Eight-week-old F-344, Wistar, and Sprague-Dawley rats received 20 microCi [3H]DFP via the jugular vein. The latter two strains served to validate the procedure as significant data exist on the erythrocyte life spans of these rats. The red cell life span of the F-344 rat has not been previously investigated. Blood samples were collected from a tail vein at 8 days post-labeling and approximately three times per week until Day 33. Whole blood samples were individually prepared for scintillation counting. Blood radioactivity, corrected for sampling loss, was expressed as a percentage of the total activity at the time of initial sampling. Data were analyzed using the least-squares method of linear regression. Results, expressed as a mean erythrocyte life span in days (+/- 1 SE), for each strain were 66.3 +/- 1.6 for F-344, 59.8 +/- 2.2 for Wistar, and 61.0 +/- 1.3 for Sprague-Dawley rats. The results for the Wistar and Sprague-Dawley rats were in good agreement with published data for these strains, thus validating the modified [3H]DFP method for erythrocyte life span determination. In addition, the red cell life span of the F-344 rat was shown to be comparable with other rat strains.