To circumvent the limitations of using postmortem brain in molecular assays, we used avidin-biotin binding to couple superparamagnetic iron oxide nanoparticles (SPIONs) (15-20 nm) to phosphorothioate-modified oligodeoxynucleotides (sODNs) with sequence complementary to c-fos and -actin mRNA (SPION-cfos and SPION-actin, respectively) (14 -22 nm). The Stern-Volmer constant for the complex of SPION and fluorescein isothiocyanate (FITC)-sODN is 3.1 ϫ 10 6 /M. We studied the feasibility of using the conjugates for in vivo magnetic resonance imaging (