Malarial gametocytes circulate in the peripheral blood of the vertebrate host as developmentally arrested intra-erythrocytic
cells, which only resume development into gametes when ingested into the bloodmeal of the female mosquito vector. The
ensuing development encompasses sexual reproduction and mediates parasite transmission to the insect. In vitro the
induction of gametogenesis requires a drop in temperature and either a pH increase from physiological blood pH (ca pH
7·4) to about pH 8·0, or the presence of a gametocyte-activating factor recently identified as xanthurenic acid (XA).
However, it is unclear whether either the pH increase or XA act as natural triggers in the mosquito bloodmeal. We here
use pH-sensitive microelectrodes to determine bloodmeal pH in intact mosquitoes. Measurements taken in the first 30 min
after ingestion, when malarial gametogenesis is induced in vivo, revealed small pH increases from 7·40 (mouse blood) to
7·52 in Aedes aegypti and to 7·58 in Anophěles stephensi. However, bloodmeal pH was clearly suboptimal if compared to
values required to induce gametogenesis in vitro. Xanthurenic acid is shown to extend the pH-range of exflagellation in
vitro in a dose-dependent manner to values that we have observed in the bloodmeal, suggesting that in vivo malarial
gametogenesis could be further regulated by both these factors.