This study assessed the reliability of131I albumin-dilution for measuring induced changes in plasma volume (PV) immediately after coronary bypass surgery. After injection of the tracer, blood samples were collected from 18 control patients to obtain131I albumin decay curves in the plasma. These curves were used to construct a mathematical model describing tracer decay. PV was then calculated in control patients, and in 51 study patients who received plasma expanders on the first postoperative morning. At 1, 15, 30, and 60 min after volume loading in the study group, dilution volumes were calculated as the difference between predicted and observed plasma radioactivity levels. These calculations were compared to an independent measurement of PV. This method was associated with a measurement error no greater than ± 12% in 82% of the 51 study patients. It tended to underestimate PV by 2.4%.