A method for continuous recording of flowing arterial blood in humans has demonstrated that injected iodinated (I131) human serum albumin disappears at variable rates from the circulation when losses are calculated from a two hour extrapolation to zero time. Losses from the circulation during the ten minute interval following injection were minimal. The recorded disappearance curve inscribed during the two hour period subsequent to mixing apparently declines as a single exponential function. In the absence of disturbed circulatory states, blood volume calculated from a single ten minute sample agreed with values obtained by extrapolation within the limits of error of counting methods.