One hundred and forty six calving interval records were built up from 64 N'Dama cows maintained for 3.5 years under a high natural tsetse challenge in Zaire. Matching health and performance data were recorded monthly to allow simultaneous evaluation of the effects of different criteria of trypanotolerance represented by time detected parasitaemic, parasitaemia score and packed red cell volume percent (PCV) on reproductive performance, calf weaning weight and cow productivity. Control of development of anaemia, measured by PCV value during trypanosome infection, had the major effect on all three performance traits. The repeatability of this criterion (0.33) was almost equal to that of calf weaning weight, indicating PCV measurement might be suitable for identification of more trypanotolerant animals. Simultaneous evaluation of the relative effects of control of development of anaemia in both the pre-weaner calf and its dam, on calf performance, suggested that its measurement in an animal might be feasible at an early post-weaner stage. Guidelines for work to develop practical field tests for trypanotolerance involving post-weaners maintained for varying lengths of time in high natural challenge situations are suggested.