The kinetics of the mercury-binding profiles of soluble proteins from the kidneys of rats receiving po phenylmercuric acetate (PMA) or mercuric acetate (Hg 2+) were investigated. PMA and Hg 2+ treatment resulted in different 203Hg-binding patterns during the first 48 hr, especially in peak 1 and 2 proteins molecular weight > 100,000 and between 40,000 and 60,000, respectively), which gradually became similar at later times. A very large portion of 203Hg was bound to a soluble protein of 8000–13,000 molecular weight (peak 3). The major characteristic of this protein is its capacity to bind mercury. Two rates of removal of PMA were observed for all kidney soluble proteins. The fast rate has a half-time of 19–22 hr; that of the slow process is 12–44 days. Only one slow rate of removal was observed with Hg 2+, and the half-times were 10, 12, and 124 days for peaks 1, 2, and 3, respectively. Rats receiving multiple doses of PMA or Hg 2+ also showed similar 203Hg-labeling profiles in the kidney soluble proteins, and a slight change of OD 260 pattern with an increase in peak 3.