Thioredoxin reduction in plant chloroplasts is catalyzed by a unique class of disulfide reductases which use a one-electron donor, [Fe 2 S 2 ] 2+,+ ferredoxin, and has an active site involving a disulfide in close proximity to a [Fe 4 S 4 ] 2+ cluster. In this study, spinach ferredoxin:thioredoxin reductase (FTR) reduced with stoichiometric amounts of reduced benzyl viologen or frozen under turnover conditions in the presence of thioredoxin is shown to exhibit a slowly relaxing S ) 1/2 resonance (g ) 2.11, 2.00, 1.98) identical to that of a modified form of the enzyme in which one of the cysteines of the active-site disulfide is alkylated with N-ethylmaleimide (NEM-FTR). Hence, in accord with the previous proposal [Staples,