In order to study protein degradation during flight in homing, a high-performance liquid chromatography technique was developed for the quantitative analysis of N tau-methylhistidine. Secondly, it was necessary to confirm that the excretion of N tau-methylhistidine correlates with myofilament breakdown in homing pigeons. In these experiments, ten birds were subcutaneously injected with N tau-[14C]methylhistidine and the excreta were quantitatively collected for 1 week. Of the 94.5% radioactivity recovered, 87.1% was associated with N tau-[14C]methylhistidine and 6.1% with N-acetyl-N tau-[14C]methylhistidine. This rapid excretion of unmetabolized N tau-[14C]methylhistidine validates the assumption that the amount of N tau-methylhistidine excreted is a measure of myofilament catabolism in homing pigeons. The influence of endurance flight on protein breakdown was determined after flights from release sites 368-646 km away. Immediately after return, plasma urea and uric acid levels were increased, whereas plasma concentration of N tau-methylhistidine remained unchanged compared to unflown control birds. Flown pigeons excreted significantly more urea and N tau-methylhistidine within 24 h and significantly more urea and uric acid within 96 h after flight than unflown controls. Our findings support the hypothesis that in homing pigeons protein catabolism is increased during endurance flight. Elevated N tau-methylhistidine excretion probably results from repair processes in damaged muscle fibers, including breakdown of myofilaments.