The aim of this study was to characterise concentration- and time-dependent responses in juvenile Atlantic cod (Gadus morhua) following exposure for one and three weeks to the water-soluble fraction (WAF) of three weathered oils: Arabian Light crude oil (ALC), North Sea crude oil (NSC) and ship-diesel. The sum of polycyclic aromatic hydrocarbons (PAH) in water was highest after one week of exposure and within environmentally relevant concentrations. PAH metabolites in bile confirmed exposure to and uptake of PAHs. Hepatic cytochrome P450 1A (CYP1A) gene expression (here and later the phrase gene expression is used synonymously to gene transcription, although it is acknowledged that gene expression is also regulated by, e.g., translation and protein stability) increased dramatically following exposure to all three oil types (fold-change up to 165), and there was a time lag between gene and protein expression. Hepatic CYP1A protein concentration and ethoxyresorufin-O-deethylase (EROD) activity were however more variable among individuals and treatments than gene expression. Hepatic and gill EROD increased in fish exposed to WAF from the two crude oils, but not in fish exposed to WAF from diesel. Exposure to diesel appeared to induce oxidative stress to a larger extent than exposure to crude oils. Other biomarkers (glutathione S-transferases, acetylcholine esterase, vitellogenin) did not appear to respond to the exposure and hence did not discriminate among oils. Biomarker responses in cod after exposure to weathered crude oils and diesel suggested the CYP1A system and oxidative stress markers to have the highest potential for discriminating among different oil types and to monitor the environmental consequences of spills.