Rabbits were infused with 3.7 GBq (100 mCi) of [24Na]Na ion in a 100-ml sodium carbonate solution. Beta particles were detected using a Tennelec Counting System; background counts were 1.6 +/- 1 counts per minute (cpm). Counts for one nanoliter of blood ranged from 22 to 30 cpm. Blood volumes on the mouthparts of tabanids following a 15-sec interrupted feeding were estimated to be 12.5 nl for Tabanus fuscicostatus, 10.8 nl for T. nigrovittatus and 6.12 nl for Chrysops fuliginosus. Estimates of the quantity of blood adhering to 22-gauge needles and insect pins (size 2) following a percutaneous intramuscular needle-stick were 8.8 +/- 1.0 nl and 5.7 +/- 1.8 nl, respectively. Mosquitoes, Aedes aegypti females, were fed to repletion with a 22Na-artificial diet, and radioactivity was measured using a Packard Autogamma 5650. The estimated average blood meal size was 2.80 +/- 0.94 microliters.