The P-glycoproteins (Pgp's) are a small family of proteins frequently associated with the multidrug resistance phenotype in drug-selected cell lines. The number of Pgp isoforms in different mammalian species is variable although the reason for having a larger or smaller number of isoforms is not known. Two isoform classes from human, and three from rodents have been extensively characterised and have been shown to have independent expression patterns and substrate preferences. We have cloned 3' terminal genomic fragments for five members of the Pgp multigene family from the pig, which is the largest number of Pgp genes found in any mammalian species to date. Sequential duplications of one class of Pgp gene have given rise to this large gene family since four genes show similarity to the drug resistance-causing Class I isoform of Pgp. The fifth pig Pgp gene shows similarity to the phosphatidylcholine-translocating Class III isoform. The history of the duplications creating this large gene family can be traced by atypical features which have been inherited in common. These include a mutation in the stop codon at the 3' end of four Class I Pgp genes, increasing the coding region by six amino acids, and a SINE element of the PRE1 family inserted into the 3' untranslated region of three Class I Pgp's. We demonstrate expression of Class I pgp in pig brain cultured capillary endothelial cells, and Class III pgp in the liver, two important sites of expression of Pgp in rodents and humans. Thus there appears to be strong phylogenetic conservation in mammals of both sequence and expression of these two Pgp isoforms.