The purpose of this study was to investigate the interrelationship between the ventilatory anaerobic threshold (VAT), the blood lactate anaerobic threshold (LAT), and the alteration in muscle metabolism. Ten subjects (5 men and 5 women) performed progressive exercise to exhaustion on two occasions for determination of the VAT and the LAT. For both AT criteria, the initial breakpoints (P less than 0.05) in the relationship between ventilation (VE) and O2 uptake (VO2VAT) and lactate (La) and power output (POLAT) were determined by multisegmental linear regression. During three subsequent tests the subjects performed progressive exercise to various percentages of the VO2VAT. Biopsies were obtained from the musculus vastus lateralis for determination of selected glycolytic intermediates at the cessation of exercise. It was found that the VO2VAT, expressed in terms of power output (POVAT), occurred at a higher value (P less than 0.05) than the POLAT (1,004 vs. 621 kg X min-1). Blood La values at these power outputs were 2.09 and 1.25 mM, respectively. Determination of the muscle La concentration at 79, 94, and 110% of VO2VAT indicated significant increases (P less than 0.05) from rest values of 1.59 to 4.49, 6.37, and 11.3 mmol X kg wet wt-1, respectively. It is concluded that the gas exchange AT as determined by the relationship between VE and VO2VAT and the AT as determined by blood La accumulation (LAT) are not coincidental. In addition the elevation in muscle anaerobic glycolysis precedes both the VAT and the blood LAT in this progressive exercise test.