The lipophilic neutral complex [99mTc][TcO(MRP20)] [MRP20 is (N-(2(1H pyrolylmethyl]N'-(4-pentene-3-one-2)ethane-1,2-diamine)] is known to cross the blood-brain barrier in non-primate animals. We report here its in vivo biodistribution, radiation dosimetry, and single-photon emission computed tomography (SPECT) characteristics in man. Following i.v. administration of 15-25 mCi of the tracer, the maximum uptake of activity in the brain peaked at 1 min p.i. Fifteen minutes p.i., the percentage retained in the brain was 5.2 +/- 1.6, which remained fairly constant over 24 hr. Blood clearance was relatively slow with an apparent affinity of the compound for the cellular fraction of the blood, however, soft-tissue and facial activity cleared at a rate four times faster than that of the brain. SPECT images obtained at 15 min, 1 hr, 3 hr, 7 hr, and 24 hr p.i. showed no redistribution of the tracer within the brain. The dosimetry is favorable for administration of 25-30 mCi of MRP20. Our results indicate that this compound is rapidly extracted and retained by the brain and may be used for SPECT imaging of regional blood flow.