To derive a formula for determination of 99mTc-DTPA clearance (Cl) from the radioactivity in a single plasma sample, the relation Cl = ECV/t was used, where ECV is the extracellular volume and t is the mean transit time of the tracer in the organism. By studying 99mTc-DTPA time activity curves of 45 consecutive patients we found that ECV could be estimated from the body surface area, and that t could be calculated from the radioactivity in a single plasma sample. Cl calculated by the single sample method was almost identical to Cl calculated by a standard multiple sample method (r = 0.987). It is concluded that the single sample method is accurate and that it may prove useful as a routine method provided that the method is not used in patients with 99mTc-DTPA clearance less than 30 ml min-1, since this investigation includes only patients with Cl greater than 30 ml min-1; the method is not used for children, since the test material did not contain any children; the method is only used for plasma samples drawn at 180 less than or equal to t less than or equal to 300 min.