A pair of monozygous twins, a trained athlete and his identical untrained counterpart, were tested over a period of 1 1/2 years. Monozygosity was established on the basis of morphological traits and serological criteria. Repeated measurements obtained during basal conditions, submaximal and maximal exercise on a cycle ergometer disclosed that: a) the basal heart rate, respiratory rate, minute ventilation, vital capacity and forced expiratory volume were insensitive to training; b) cardiorespiratory displacement during submaximal work was smaller in the trained than in the untrained twin; and c) the maximum muscular force, mechanical power output, aerobic power and anaerobic capacity were about 10, 60, 37 and 60%, respectively, higher in the trained than in the untrained twin. The increase of the maximal aerobic power was attributed equally to an enhancement of the O2 transport and O2 utilization systems. It was concluded that although exogenous factors are important, a strong hereditary component sets a limit to functional adaptability.