The electrical conductance of ds-DNA duplexes containing 8-14 base pairs modified at both ends with a -(CH2)(6)-SH linker was measured in a buffered aqueous solution using electrochemically controlled distance tunneling spectroscopy. The tunneling experiment with self-complementary 5'-(GC)(n)-3'-(CH2)(6)-SH (n = 4-7) duplexes attached covalently to a gold STM tip and a Au(111) electrode shows a wide distribution of currents independent of the ds-DNA length. The voltage-induced horizontal orientation of ds-DNA within the junction results in decreased electrical conductance. The lower currents are also observed for ds-DNA molecules containing a single CA base mismatch.