Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.
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