We present here the results of an ab initio non-equilibrium Green's function study of the electron transport properties of the porphyrin molecule located between gold electrodes. One-electron transmission spectra were calculated for two different orientations of porphyrin molecule: 'diagonal' and 'orthogonal'. It is shown that porphyrin exhibits significant conductance at small voltage biases, displaying an overall conductance for the 'diagonal' conformation that is higher than conductance for the 'orthogonal' one. The importance of performing spin-unrestricted calculations is demonstrated and the influence of the structure of gold electrodes on the conductance of the porphyrin molecule is discussed. (C) 2007 Elsevier B.V. All rights reserved.