The present study examined muscle adaptations and alterations in work capacity in endurance-trained runners as a result of a reduced amount of training combined with speed endurance training. Seventeen runners were for a 6- to 9-wk period assigned to either a speed endurance group with a 25% reduction in the amount of training but including speed endurance training consisting of 6-12 30-s sprint runs 3-4 times a week (SET, n=12) or a control group (CON, n=5), which continued the endurance training (about 55 km(.)wk(-1)). For SET the expression of the muscle Na(+)/K(+) pump alpha2-subunit was 68% higher (P<0.05) and plasma K(+) was reduced (P<0.05) during repeated intense running after 9 weeks. Performance in a 30-s sprint test and the first of the supra-maximal exhaustive runs was improved (P<0.05) by 7% and 36%, respectively, after the speed endurance training period. In SET, VO2-max was unaltered, but the 3-K (3,000 m) time was reduced (P<0.05) from 10.4+/-0.1 (mean+/-SEM) to 10.1+/-0.1 min and 10-K (10,000 m) time was improved from 37.3+/-0.4 to 36.3+/-0.4 min. Muscle protein expression and performance remained unaltered in CON. The present data suggest that both short- and long-term exercise performance can be improved with a reduction in training volume if speed endurance training is performed, and a role of the Na(+)/K(+) pump in the control of K(+) homeostasis and in the development of fatigue during repeated high-intensity exercise. Key words: Fatigue, Running economy, Performance, Potassium.