Agents used to measure glomerular filtration rate (GFR) give a biexponential plasma disappearance curve on multiple peripheral venous sampling between 20 min and 4 h after intravenous injection. These two exponentials are generally regarded to represent equilibration of agent throughout the extracellular fluid (ECF) space and renal clearance, respectively. In seven subjects undergoing diagnostic arteriography, arterial and antecubital venous plasma samples were obtained up to 60 min in five and up to 120 min in two following simultaneous intravenous injection of 99Tcm-diethylene triamine pentaacetate (99Tcm-DTPA) and inulin. The count rate from 99Tcm was simultaneously recorded over the calf with a collimated scintillation probe in five subjects up to 60 min post-injection. The arterial and venous time-concentration curves were interpolated and subtracted to give a curve of the arterio-venous (A-V) concentration difference, which was then integrated. Arterial time-concentration curves display three exponentials, the first of which has the largest amplitude and disappears by about 20 min. The A-V concentration difference becomes zero at about the same time. The integral of the A-V concentration difference, which represents activity in the interstitial space of the forearm, has a time course consistent with the second compartment of a model of two compartments in series (the first being plasma) and a time course that is reciprocally similar to the first exponential of the triexponential arterial plasma curve. The curve externally recorded by scintillation probe has a shape consistent with a signal that is the composite of interstitial 99Tcm-DTPA and plasma 99Tcm-DTPA activities. The arterial plasma clearance curve of GFR agents is triexponential; the first exponential reflects equilibration of agent between plasma and the interstitial space of carcass tissue (mainly muscle and skin). The second exponential is minor compared with the first; it is not clear what it represents. The third exponential reflects renal clearance.