Cardiac performance in moderate bicycle exercise has been measured in five groups of New Guineans (N = 161). Men and women in both coastal and highland (2000 m) locations were studied, as well as a group of highland men after migration to the coast. The results have all been standardized to an oxygen consumption of 1 1.min(-1). Cardiac output values are similar to those previously quoted for Europeans; a significant positive correlation with body size is found, and resident highlanders appear to have slightly larger cardiac outputs (+4%) and larger stroke volumes (+10%) after adjustment for body size, but this is not because they have larger hearts. The highland migrants at sea-level share none of the 'altitude' characteristics of the highland residents and, after size-adjustment, correspond with the lowlanders. The cardiac frequencies of the lowland women are higher than the other groups (+15 beats.min(-1)) after adjustment for differences in body size. Their values are close to those found previously for moderately active Europeans, while all the other New Guinean subjects have lower cardiac frequencies probably because of their greater habitual activity.