Pleiotropic drug resistance in the yeast Saccharomyces cerevisiae results mainly from the overexpression of genes encoding membrane efflux pumps, the so-called ABC and MFS transporters. These pleiotropic drug resistance loci are under the control of the key transcription factors Pdr1p and Pdr3p. We have identified and characterized several new domains of Pdr1p. By testing a series of LexA-PDR1 derivatives for their capacity to activate a GAL1-lacZ reporter gene we have shown that the C-terminal domain of Pdr1p comprising amino acids 879-1036 is involved in transcriptional activation, and that the point mutation pdr1-8 increases its efficiency. Removal of amino acids 1006-1029, which include a polyasparagine stretch, decreases the activation function. Internal deletions within Pdr1p reveal the presence of a large regulatory domain, and a short but strong inhibitory subdomain spanning amino acids 257-316, in which the up-regulating mutations pdr1-2, pdr1-6 and pdr1-7 are located. A mini-Pdr1p consisting of only the DNA-binding and the activation domains strongly up-regulates the expression of the major target genes PDR5, SNQ2 and YOR1, resulting in enhanced multidrug resistance.