The trypanosomes multiply during prepatency after minimal infection by a factor of 2.2-3.6 per day. During patency, increase of trypanosome number in the peripheral blood is basically non-logarithmic as the actual proliferating forms remain hidden in special vascular areas (kidney, placenta). The mean increase during patency is approximately linear, typically by 1-10 trypanosomes per 10(4) erythrocytes per day, and depends on the number of infective organisms introduced. - Length of parasitemia is 13-40 days, it is partly determined genetically. Prolonged parasitemias, very low in the later stages, were seen in mice preinfected with Trypanosoma congolense. - In 1 mouse with minimal infection, parasitemia only became patent 63 days after infection under the stress of an added infection with Trypanosoma brucei. - Parasitemia can be terminated by antiserum. Application of antiserum 3 or 4 days after infection may even suppress the development of immunity and priming. - Massive reinfection leads to parasitemia of up to 8 days duration without parasite multiplication.