In the bovine serum anaphylaxis of the dog the histamine content in blood plasma was increased about 33 times and in the thoracic lymph 40 times that before reinjection. In the dogs injected with 10 mg/kg Benadryl 30 minutes prior to reinjection the maximum increase of the plasma histamine was only 4.5 times and that of lymph histamine also only 4.5 time (the average of three cases in each). In the unprotected sensitized dogs the histamine content of the liver was reduced by 54 per cent and about 6 mg of histamine was depleted of the whole liver, and 29 per cent of the skin histamine and 1 mg histamine reduced from the whole skin, under a similar shock. Histamine release from the skeletal muscle could not be recognized in any dog. In Benadrylprotected dogs 17.4 per cent of histamine was released from the liver; 2.2 mg from the whole liver; 14 per cent from the skin; and 0.3 mg from the whole skin (the average of 3 cases in each). These results support the contention that Benadryl inhibits the union of antibody (precipitin) and antigen in the dog in anaphylaxis as reported in the preceding part of this series of studies.