The purpose of this study was to examine whether speed endurance training (SET, repeated 30-s sprints) and heavy resistance training (HRT, 80-90% of 1 repetition maximum) performed in succession are compatible and leads to performance improvements in moderately trained endurance runners. For an 8-week intervention period (INT) 21 male runners (maximum oxygen uptake (VO2-max): 59±1 mL/min/kg; mean ± SE) either maintained their training (CON, n=11) or performed high intensity concurrent training (HICT, n=12) consisting of two weekly sessions of SET followed by HRT and two weekly sessions of aerobic training with an average reduction in running distance of 42%. After 4-weeks of HICT, performance was improved (P<0.05) in a 10-km run (42:30±1:07 vs. 44:11±1:08 min:s) with no further improvement during the last 4 weeks. Performance in a 1500-m run (5:10±0:05 vs. 5:27±0:08 min:s) and in the Yo-Yo IR2 test (706±97 vs. 491±65 m) improved (P<0.001) only following 8 weeks of INT. In HICT, running economy (189±4 vs. 195±4 ml/kg/km), muscle content of NHE1 (35%) and dynamic muscle strength was augmented (P<0.01) after compared to before INT, whereas VO2-max, muscle morphology, capillarization, content of muscle Na(+)/K(+) pump subunits and MCT4 were unaltered. No changes were observed in CON. The present study demonstrates that SET and HRT, when performed in succession, leads to improvements in both short- and long-term running performance together with improved running economy as well as increased dynamic muscle strength and capacity for muscular H(+) transport in moderately trained endurance runners.