Bloodstream forms of Trypanosoma brucei brucei (EATRO 110) were cultured with 100 µM difluoromethylornithine (DFMO). After 48 hr, intracellular putrescine was depleted and cells were positive when histochemically stained for the mitochondrial marker enzyme, NAD diaphorase, and exhibited mitochondrial proliferation and cristae development when examined by electron microscopy. This suggested that the mitochondrion was undergoing the physiological transformation necessary for successful transmission of the bloodstream form to the vector, namely the initiation of development of a TCA cycle and cytochrome system. The short stumpy forms that appeared by day 4 of culture, although physiologically transformed, were not viable in so far as they were not capable of transforming to procyclic trypomastigotes when introduced into SDM-79 medium. When rats infected with T. b. brucei were given 4% (w/v) DFMO in their drinking water, they were cured within 72 hr. Trypanosomes removed from animals and stained for NAD diaphorase showed mitochondrial transformation, as well as an intermediate and short stumpy morphology, at 36 and 60 hr, respectively. Data from this study on the growth and transformation characteristics of the DFMO induced intermediate and short stumpy form trypanosomes supports the observation that the intermediate form, and not the short stumpy form, is able to successfully transform to procyclic trypomastigotes.