Seven men performed one-legged isometric knee extension at 5% MVC for 1 h. Total body oxygen uptake amounted to 451 (420-471) ml min-1 and oxygen uptake over the contracting leg to 200 (172-216) ml min-1, with no changes occurring during the 1 h contraction. Venous O2 tension decreased from 29.4 mmHg at rest to 23.1 mmHg with contraction and CO2 tension tended to increase from a resting value of 50.5 mmHg to 57.2 mmHg (n.s.). No similar changes occurred in arterial O2 and CO2 tensions. There was a small but continuous glucose uptake at both rest and throughout the contraction, whereas a lactate release occurred only in the early phase (2 min) of contraction. Muscle glycogen content was 312 mmol kg-1 dry wt at rest, no significant changes had occurred following 30 min or 1 h of contraction. Arterial and venous Hct and Hb values indicated that a flux of water occurred from the vascular bed to the contracting muscle, in which H2O increased from 3.06 l kg-1 dry wt at rest to 3.30 l kg-1 dry wt after 1 h at 5% MVC. Simultaneously potassium (K), was released from the muscle throughout contraction with a mean venous-arterial difference of 0.25 mmol l-1. With a plasma flow of 335 ml min-1 kg-1 wet wt the K loss amounted to 5 mmol kg-1 wet wt or roughly 5% of the total muscle K content.(ABSTRACT TRUNCATED AT 250 WORDS)