Recent studies with Leishmania donovani have indicated that transformation from leishmania to leptomonad in culture involves a shift to aerobic metabolism. An attempt was made to confirm this hypothesis by measuring changes in parasite respiratory activity corresponding to body shape during transformation. Parasites were prepared from infected rodent spleens by homogenizing and centrifugation; preparations were diluted to contain a constant number of organisms, one-half volume of a saline-citrate-hemolyzed blood medium was added, and O 2 uptake was measured at 28 °C on a differential respirometer for up to 27 hours. Stained smears were made periodically to monitor transformation. Leishmanias exhibited a falling respiratory rate for the first 3 to 6 hours and a steadily increasing respiratory rate thereafter as transformation progressed. The initial fall in O 2 uptake was attributed to “dying” host tissue contaminant, since uninfected spleens prepared as above showed the initial fall in respiration but did not show the subsequent increase.