Normal plasma contains inactive renin, which becomes active when plasma is dialyzed to pH 3.3 and to pH 7.5, or treated with pepsin or trypsin. Under optimal conditions, each of these procedures activated the same quantity of renin, which was not further increased by repeating or combining two procedures, thus suggesting that the same pool of inactive renin was activated by each procedure. When plasma was fractionated by gel filtration, dialysis activated very little renin in eluates. Trypsin activated renin, but under some conditions also destroyed renin. Pepsin fully activated the inactive renin in eluates without evidence of destruction of renin. The pepsin-activated renin of normal plasma eluted from Sephadex G-100 in a peak of apparent molecular weight (MW) 58,000 and from Sephacryl S-200 with apparent MW 53,000, like big renin in plasma of patients with diabetic nephropathy. Inactive renin was usually increased in amount in plasma of sodium-depleted normal men, but the elution volume did not change with sodium intake. When renin was fully activated in plasma incubated with pepsin or trypsin, the apparent MW of the main peak of big renin did not change appreciably. Inactive renin in plasma was usually increased after sodium depletion, but the elution volume did not change. Active renin of normal plasma had an apparent MW near 41,000 on both gels. Thus, we conclude that big renin is present in normal plasma in amounts at least equal to and usually greater than active renin (the ratio depending on sodium intake) and that pepsin activation readily demonstrates big renin in eluates from gel filtration.