The substitution of rhenium and ruthenium for manganese in the perovskite CaMnO3 significantly lowers the resistivity in the low temperature region and simultaneously induces ferromagnetism, and, consequently, a colossal magnetoresistance effect is observed. This behavior is explained in terms of valency effect: Re and Ru are mainly pentavalent, thus their substitution for Mn generates Mn3+ species, allowing double-exchange interaction between Mn3+ and Mn4+. Thermoelectric power measurements confirm this model, showing a shift from -350 muV.K-1 for the undoped phase, to -100 muV.K-1 for the doped phase (6% Re or Ru) at 300 K. (C) 2000 Elsevier Science Ltd. All rights reserved.