The rates of elimination of iodinated human serum albumin (ALBUMIN-125I), polyvinylpyrrolidone (PVP-125I); and tritiated dextran (DEXTRAN - METHOXY-3H) (mol masses of 69 000 - 72 000, 30 000 - 40 000 and 60 000 - 90 000 respectively) from the circulation of sheep were studied; albumin and PVP initially disappeared from the circulation rapidly having half-life times (t 1/2) of 10,5 +/- 3,7 and 43 +/- 45 hours respectively. This phase is regarded as being due to equilibration within the initial volume of distribution, the rate being determined primarily by the relative mol. mass of the molecule. Hereafter PVP-125I was eliminated considerably faster (t 1/2 = 176 +/- 39 h) principally via the kidneys. The limited data available for dextran-3 H suggests that this particular substance is rapidly excreted via the kidneys (t 1/2 = 9 h).