The present study examined NH3 (ammonia and ammonium) uptake in resting leg muscle. Six male subjects performed intermittent arm exercise at various intensities in two separate 32-min periods (part I and part II) and in one subsequent 20-min period in which one-legged exercise was also performed (part III). The arterial plasma NH3 concentration was 79.6 +/- 9.6 (SE) mumol/l at rest and 88.1 +/- 9.1, 98.1 +/- 8.1, and 210.2 +/- 7.5 mumol/l after 10 min of exercise in parts I, II, and III, respectively. The corresponding NH3 uptakes in the resting leg were 3.3 +/- 1.3 (rest), 7.8 +/- 1.5, 14.0 +/- 4.5, and 57.7 +/- 18.3 mumol/min. Throughout each exercise period a net uptake of NH3 was observed in the resting leg (P < 0.05), but uptake decreased to resting values within 5 min of termination of exercise. The muscle NH3 concentration of 195.1 +/- 15.0 mumol/kg wet wt at rest was largely unchanged throughout the experiment. The present data suggest that resting muscles extract NH3 and contribute significantly to clearance of NH3 during exercise and in early recovery from exercise. The extracted NH3 appears to be metabolized within the resting muscles.