WHILE testing the effects of insulin on the circulatory reaction of laboratory rats to dextran, we found that many animals died after combined doses of the two agents which were not lethal when administered separately. The validity of this observation was put to experimental test. Male Sprague-Dawley rats weighing at least 150 gm. and fasted 24 hr. were sub-cutaneously injected with zinc insulin (‘Iletin’) at two dose-levels, 7.5 or 10 U./kgm. body-weight. An hour later dextran was administered by vein in a total volume of 10 ml./kgm. 0.85 per cent sodium chloride containing either 60 or 600 mgm. dextran (‘Expandex’). Control animals received saline without dextran. Fatalities due to these amounts of dextran have not occurred in our work. The results of eight experiments are summarized in Table 1, which shows the incidence of survival after 24 hr. Animals that died exhibited convulsions usually within 3 hr. of insulin administration. Few signs of the dextran reaction characteristic of this species were noticed.