There is a requirement for the characterisation of the metabolism of xenobiotics in zebrafish larvae, due to the application of this organism to toxicity testing and ecotoxicology. Genes similar to mammalian cytochrome P450 (CYP) 1A1, CYP2B6, CYP3A5 and UDP-lucuronosyl-transferase (UGT) 1A1 were demonstrated to be expressed during normal embryonic development, with increased expression post hatching in Wik strain zebrafish larvae (72 hours post fertilisation, hpf). Activities towards ethoxy-resorufin, 7-ethoxy-coumarin and octyloxymethylresorufin, using an in vivo larval assay, were also detected in 96 hpf Wik strain zebrafish larvae, indicative of oxidative and conjugative metabolism. The expression of the identified genes was modulated upon exposure to Aroclor 1254, and the metabolic activities towards ethoxy-resorufin, 7-ethoxy-coumarin and octyloxymethylresorufin were observed to be inducible by exposure to in vitro inhibitors of CYP activities. Wik strain zebrafish larvae (72 hpf) were also demonstrated to metabolise the pharmaceuticals acetaminophen and ibuprofen by oxidative and conjugative processes using liquid chromatography mass spectrometry and scintillation counting. Finally, the organic solvents dimethyl-sulfoxide and methanol were observed to reduce the expression of CYP and UGT genes, and the metabolism of ethoxy-resorufin, after 24 hours exposure to \(\leq\)0.1% volume/volume concentrations of the two solvents. It is suggested that these inhibitory effects are in part due to a reduction in the expression of the aryl hydrocarbon receptors, which are known regulators of drug metabolism genes. Overall the expression of genes and enzymatic activities similar to the mammalian drug metabolism genes have been demonstrated in Wik strain zebrafish larvae (96 hpf), and the first examples of the metabolism of pharmaceuticals by zebrafish larvae are also demonstrated. The modulation of the metabolism of xenobiotics by organic solvents suggests that caution must be exercised when interpreting data from toxicity tests when high solvent concentrations are applied to zebrafish larvae. The continued use of zebrafish larvae as a toxicity testing model is strengthened by the findings of this work.