1. The emergence of the gametocytes of Plasmodium gallinaceum from the erythrocytes and the exflagellation of the male gametocyte were inhibited completely when infected blood was exposed to an atmosphere of 5 % CO2 and 95 % O2, or 5 % CO2 and 95 % N2. When these gas mixtures were replaced by air, the gametocytes emerged from the erythrocytes and the male gametocytes exflagellated after approximately the same interval as they did in newly drawn blood exposed to air, or in blood ingested by mosquitoes. If pure CO2 were used instead of the gas mixtures containing 5% CO2, the development of the gametocytes, after the replacement of the gas by air, was uncertain.
2. The emergence of the gametocytes and exflagellation of the male gametocytes proceeded normally when parasitized blood was exposed to an atmosphere of N2.
3. The pH of freshly drawn normal chick blood was 7·38–7·46. The pH of blood exposed to air rose over a 20 min. period of observation, the highest pH recorded being 7·97. No significant difference in pH was observed between normal and heavily infected blood. It was not possible to correlate the stages of development of the gametocytes with precise values of pH of the blood.
4. The development of the gametocytes proceeded normally when parasitized blood was mixed with isotonic alkaline Tris buffer to give a pH of approximately 8·0; but it was inhibited, though not completely, when the pH was reduced below that of freshly drawn blood by the addition of isotonic acid buffer. The inhibitory effect of the acid buffer could be completely but not invariably reversed by alkaline buffer.
5. 5 % CO2 had no inhibitory action upon the gametocytes if the pH of the blood-buffer mixture was 7·62, but when it was 7·20 the inhibitory effect of the gas was complete.
6. The emergence of the gametocytes from the corpuscles and the exflagellation of the male gametocytes were completely inhibited when washed, parasitized corpuscles were suspended in isotonic buffer pH 7·78 or 8·20, but this inhibition was completely reversed when the corpuscles were resuspended in plasma. It is concluded that a factor present in plasma is necessary for the development of the gametocytes.
7. Further evidence that the pH of the blood, alone, is not the only factor affecting the emergence of the gametocytes and exflagellation of the male gametocytes was obtained from birds treated intravenously with Diamox. When blood from such birds was exposed to air the gametocytes emerged from the corpuscles and exflagellation took place, although the pH of the blood was as low as, or lower than, that of freshly drawn blood.
8. The emergence of the gametocytes from the erythrocytes and exflagellation of the male gametocytes were not stimulated by secretions from the stomachs or salivary glands of gorged or unfed mosquitoes.
9. The lowest measurement of pH of blood from mosquitoes' stomachs was 7·47 and the highest 7·90, the mean being 7·68, which is slightly higher than that of freshly drawn chick blood. No correlation could be found between the pH of the blood and the length of time since its ingestion by the mosquito.