Organic field-effect transistors (OFETs) using metal complexes, bis(4-methyl-1,2-phenylenediamino) nickel (1) and bis(dithiobenzyl) nickel (2) were fabricated, and the transistor characteristics were measured in air and under vacuum. Compound I is a strong donor and oxidized at the redox potential of 0 V, while 2 is an acceptor and reduced to an anion around the same redox potential. Compound 1 shows p-type characteristics, whereas 2 exhibits n-type characteristics. Atmospheric oxygen increases both the 'on' and 'off' currents in p-type transistors of 1. The n-type characteristics of 2 are maintained in a few minutes in air, but degenerated in a few hours. (c) 2006 Elsevier B.V. All rights reserved.