A dre na lin w a s used to investigate in vivo the circulation o f the different stages of rodent Plasm odium present in the blood. A single dose o f adren alin injected to m ice infected w ith P. yoelii resulted im m ediately in i) a dim inution o f the parasitaem ia of a p pro xim ate ly 5 0 % in the peripheral la rge vessels (estimated in tail b lo o d films), as w ell as in the ca p illa rie s (estimated in smears o f b lo o d collected from a fed A nopheles), an d ii) an increased

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... m ia in b lo o d collected b y c a rd ia c puncture from the right heart. The numbers o f young stages o f P. y o e lii in the peripheral blo o d w ere initially som ew hat reduced but, unexpectedly, midterm trophozoites w ere preferentially expelled from the peripheral blood into m ajor organs like the heart. W ith P. vinckei, parasitaem ia decreased on ly w he n midterm trophozoites predom inated, and w ith P. c h a b a u d i no effect w a s observed at a n y time. W e propose that midterm trophozoites, by their increased surface area, as co m p a re d to rings, an d their flexibility w hich contrasts w ith the rig id schizonts, are particularly susceptible to haem odynam ic perturbations.