The sensitivity to chloroquine according to the degree of synchronicit
y of Plasmodium yoelii nigeriensis, which is considered to be the most
resistant of the rodent malaria strains, was studied. The infection w
as synchronised by means of a Percoll-glucose gradient which separates
rings and young trophozoites from other stages. The mid term trophozo
ite, when it predominated in the blood at the time of treatment, was s
hown to be as sensitive to chloroquine as Plasmodium vinckei petteri.
According to previous results indicating that part of the population o
f merozoites is latent and penetrates around midnight, the inoculation
s were timed in order to obtain a lower or higher degree of synchronis
ation. The infection appeared to be better synchronised if rings and y
oung trophozoites, were inoculated at 06:00 hrs rather than at 15:00 h
rs and consequently the efficacy of chloroquine was higher in the farm
er than in the latter.