The influence of the degree of coupling between the breathing and cycling rhythms (K) on oxygen uptake (Vo2) was examined in 30 volunteers. They cycled on an ergometer with a load equal to 50% of their work capacity 170 in two experimental runs with spontaneous breathing rhythm, and in a further two runs with acoustically triggered breathing. K was continuously ascertained. Vo2 and other respiratory parameters were measured by an automatic "breath-by-breath analysis" system. In 16 subjects, Vo2-differences between runs were correlated with the differences in K. In the majority of these subjects (12), Vo2 decreased significantly with increasing K. In 14 subjects, Vo2-and K-variations within individual runs were analyzed. Phases with higher K were regularly accompanied by a decrease in Vo2. It is concluded that coupling the breathing and cycling rhythms reduces Vo2 for a given moderate work load, although the magnitude of the Vo2-reduction varies considerably between individuals.