Transmission of electrons through orbitals of molecules is discussed using high-level ab initio methods that allow an orbital interpretation. The transmission is computed for a molecule tethered between gold atoms. Strong coupling to the gold is achieved using a sulfur atom at each end of the molecule. sigma-type orbitals can conduct as well as conjugated bridges provided that the orbitals have weights on the sulfur atoms. The current depends exponentially on the voltage applied to the molecule and this voltage can also shift the electronic density and further alter the transmission. The conductivity can also be modified by a gate voltage. (C) 2003 Elsevier B.V. All rights reserved.