The multidrug resistance (MDR1) P-glycoprotein functions as a broad specificity efflux transporter of structurally diverse natural product and xenobiotic compounds. P-glycoprotein also is an important component of the functional blood−brain barrier. To enable further studies of function and modulation of MDR1 P-glycoprotein in vitro and in vivo, two novel phosphine technetium(III) complexes were designed and characterized:  trans-[2,2‘-(1,2-ethanediyldiimino)bis(1,5-methoxy-5-methyl-4-oxo-hexenyl)]bis[methylbis(3-methoxy-1-propyl)phosphine]Tc(III) (Tc−Q58) and trans-[5,5‘-(1,2-ethanediyl diimino)bis(2-ethoxy-2-methyl-3-oxo-4-pentenyl)]bis[dimethyl(3-methoxy-1-propyl)phosphine)]Tc(III) (Tc−Q63). In human drug-sensitive KB 3-1 cells and multidrug-resistant KB 8-5 and 8-5-11 derivative cell lines, expressing nonimmunodetectable, low, and high levels of MDR1 P-glycoprotein, respectively, accumulation of Tc−Q58 and Tc−Q63 was inverse to expression of the transporter. Differences between drug-sensitive and multidrug-resistant cells, while detectable at picomolar concentrations of each radiopharmaceutical, were independent of tracer concentration. Ratios of tracer accumulation in KB 3-1 and 8-5 cells were 62.3 and 48.1 for Tc−Q58 and Tc−Q63, respectively. Cell contents of Tc−Q58 and Tc−Q63 were enhanced up to 60-fold in MDR cells by known modulators of MDR1 P-glycoprotein, while drugs not in the multidrug-resistant phenotype had no effect on their accumulation. In KB 8-5 cells, potency of modulators was GF120918 ≫ cyclosporin A > verapamil. Accumulation of Tc−Q58 and Tc−Q63 in Sf9 insect cells infected with a recombinant baculovirus containing human MDR1 P-glycoprotein was reduced in a GF120918-reversible manner (EC₅₀ ≤ 70 nM) compared with cells infected with a wild-type baculovirus. By contrast, cell contents of Tc−Q58 or Tc−Q63 in Sf9 cells expressing the homologous MDR3 P-glycoprotein did not differ from wild-type virus. Demonstrating molecular targeting of these complexes in vivo, distribution and retention of Tc−Q58 in brain tissue of FVB mice treated with a saturating dose of GF120918 and mice deficient in the mdr1a gene [mdr1a (−/−)] were enhanced 180% and 520% over control, respectively. Exploiting the gamma-emission spectrum of ^99mTc, increased uptake of Tc−Q58 in brain tissue of mdr1a (−/−) mice was readily detected noninvasively by scintigraphic imaging. Thus, both Tc−Q58 and Tc−Q63 are demonstrated to be substrates for transport by MDR1 P-glycoprotein, broadening the specificity of this transporter to include phosphine-containing metal complexes. As shown with Tc−Q58, these Q complexes can be used to detect transport activity and modulation of MDR1 P-glycoprotein in vitro and to directly monitor the functional status of P-glycoprotein at the blood−brain barrier in vivo.