The use of cuprous ion as a reducing agent for pertechnetate is described for the first time. The potential usefulness of a 99mTc-Cu-DTPA complex as a renal function agent was evaluated by organ distribution studies in animals and by simultaneous continuous-infusion and single-injection clearance studies. Radioiodinated iothalamate was used as a standard for the measurement of GFR. The ratio for the clearance with the new copper complex to that with iothalamate is close to unity, both for continuous infusion (0.97 +/- 0.01 s.e) and for single injection (1.05 +/- 0.03). The coefficients of correlation are similarly close to unity: 0.96 and 0.99, respectively. Dialysis of plasma samples obtained after injection of 99mTc-Cu-DTPA into bilaterally nephrectomized rats indicates that only small amounts (less than 4%) of the complex are protein-bound. These data and the low toxicity of the copper preparation in animals appear to justify its investigation for kidney function measurement and imaging in man as well as its use as a GFR agent in animal studies.