A number of studies have been carried out on the effect of several of the 8-aminoquinolines on gametogony and sporogony of Laverania falciparum, but only very limited work has so far been done on the effect of single doses of primaquine. The authors have sought to repair this omission with a study of 12 subjects showing gametocytes of L. falciparum who were treated with a single dose of 15 mg, 30 mg or 45 mg of primaquine base, according to age. Batches of Anopheles gambiae were fed on these gametocyte carriers before and up to 7 days after treatment.It was found that a single dose of primaquine clears the blood of crescents in 4-8 days. In 5 out of 8 cases no gland infections were seen in A. gambiae fed on the first day after treatment; in 3 cases, however, sporozoites were found. After the first day following treatment no sporozoite infection of A. gambiae occurred in the 3 cases in which gland dissections were made. Feeding oöcyst-infected A. gambiae on a subject having received primaquine 3 and 4 hours before the feed had no effect on the development of the parasite.Following treatment, gametogony was apparently normal but oökinetes did not penetrate the gut wall, showed staining differences after 24 hours, and appeared to be dead after 48 hours.