1. Simultaneously obtained arterial and venous plasma atrial natriuretic factor (ANF) concentrations were compared at supine rest and during graded dynamic leg exercise in 10 healthy male subjects (aged 33–51 years).
2. Arterial ANF concentrations ranged between 12 and 179 pg/ml and venous concentrations between 9 and 177 pg/ml.
3. A positive correlation between arterial and venous concentrations was found (r = 0.984).
4. Arterial ANF concentrations were higher than venous concentrations in all pairs of samples (n = 31), but the difference was small and changed little with exercise: the mean difference was 5 pg/ml at rest, 12 pg/ml during submaximal exercise and 6 pg/ml during maximal exercise.
5. The extraction ratios for ANF varied greatly, but were in general lower (P < 0.05) during maximal exercise (median 0.07, range 0.01–0.32) than at rest (median 0.22, range 0.05–0.33).
6. It was concluded that the plasma ANF concentration in a peripheral arm vein is a good indicator of the systemic peptide concentration at rest as well as during dynamic leg exercise.