The effect of Trypanosoma brucei and T vivax on the ejaculate of sheep and the rate of recovery with diminazene aceturate treatment was investigated. Animals were made to ejaculate once a week before infection, during infection and after treatment. Parameters studied were semen volume, semen colour, sperm motility and concentration, percentage dead spermatozoa and sperm morphology. Both infections severely affected the quality of the ejaculate although in T vivax infection there was no appreciable change in semen volume and sperm concentration. There was a reduction in sperm motility and concentration, an increase in the percentage of dead spermatozoa and in abnormal sperm morphology. Treatment led to improvement in all parameters but recovery was faster in T vivax than in T brucei infection. Ejaculate with few primary abnormalities was not obtained until 19 weeks after treatment.