TOL plasmid pWWO of Pseudomonas putida contains two operons that specify a pathway for the degradation of aromatic hydrocarbons. The 'upper' operon encodes enzymes for the oxidation of toluene to benzoate and xylenes to toluates, whereas the meta-cleavage operon specifies the further oxidation of benzoate and toluates. Transcription of the upper pathway operon is positively regulated by the XylR protein, which is activated by toluene/xylenes and their alcohol catabolic products, in combination with the NtrA protein, a sigma factor. Expression of the meta-operon is positively controlled by the XylS protein which is activated by meta-pathway substrates, and is independent of NtrA protein. Expression of the meta pathway is also induced by toluene/xylene-activated XylR protein via a cascade regulatory system in which this protein in combination with NtrA protein stimulates transcription from the xylS gene promoter. Hyper-production of XylS protein in turn provokes high level expression of the meta-operon, which is independent of meta-pathway substrates. The two promoters, which are activated by the XylR and NtrA proteins, the upper pathway promoter and the xylS gene promoter, exhibit three regions of homology centred at -12(5'-TTGCATG-3'), -24(5'-TGGCPuT-3') and -45(5'-TAAAATAAGPuPuCGPuTC-3'), with respect to their principal transcription initiation points. The possible physiological significance of activated XylR-protein-induced expression of the meta-operon through amplification of XylS protein levels is considered.