The effects of histamine in malaria.

1. Extracts of the blood of monkeys (Macaca mulatta) infected with Plasmodium knowlesi contain histamine. A mean total concentration of 0.15 mug/ml was present in the circulating blood.2. No histamine was detected in the blood of healthy monkeys.3. Vasodilatation and increased vascular permeability was observed at the site of injection of the extract into the skin and brain of guinea-pigs.4. Infiltration of the dermal layer by leucocytes was observed after intradermal injection of the histamine extract. A similar response was obtained in the brain.5. The extract of the blood from the infected animal produced hypotension in rabbits when administered intravenously.6. The pathophysiological significance of histamine in malaria is discussed.