In May, 1907, we* showed that the study of the parasite of sleeping sickness (Trypanosoma gambieme, Dutton), as it appears in the blood of rats artificially infected with the disease, revealed a cyclical metamorphosis, and that this cyclical metamorphosis corresponded closely to the alternating phases of presence and absence of trypanosomes in the blood. At the same time it was found that the cyclical metamorphosis in the parasites corresponded less closely, but still unmistakably corresponded, with the successive alternations of condition that characterise the clinical aspects of the malady in the host. The features of the life-cycle of the parasites of sleeping sickness as they appear in the blood during infection in rats are remarkable, and may be briefly repeated for reference.