Changes in plasma levels of atrial natriuretic peptide (ANP) and albumin, and 
blood hematocrit (Hct) during treadmill exercise were studied in 6 healthy men 
before and after beta-adrenergic blockade. Plasma ANP levels increased during 
exercise and then gradually decreased. There was a concomitant increase in both 
Hct and plasma albumin concentrations. Prior administration of a long-acting 
propranolol, 160 mg daily for 3 consecutive days, markedly elevated plasma ANP 
levels before, during and after exercise. In addition, the mean basal Hct 
increased significantly and further rose during exercise after propranolol 
administration. When increments in plasma ANP concentrations during exercise in 
individual subjects were compared with those in Hct, there was a significant 
positive correlation between the two variables before and after propranolol 
administration. The results indicate a close relationship between the changes in 
plasma ANP and those in Hct during exercise at different ANP levels, and suggest 
that ANP may be at least one of the factors involved in the hemoconcentration 
associated with exercise.