Ultrafast emission and absorption spectroscopies were used to measure the kinetics of DNA-mediated electron transfer reactions between metal complexes intercalated into DNA, In the presence of rhodium(III) acceptor, a substantial fraction of photoexcited donor exhibits fast oxidative quenching (>3 x 10(10) per second). Transient-absorption experiments indicate that, for a series of donors, the majority of back electron transfer is also very fast (similar to 10(10) per second). This rate is independent of the loading of accepters on the helix, but is sensitive to sequence and pi stacking. The cooperative binding of donor and acceptor is considered unlikely on the basis of structural models and DNA photocleavage studies of binding. These data show that the DNA double helix differs significantly from proteins as a bridge for electron transfer.