The effects of piperonyl butoxide (PBO) and -naphthoflavone (BNF) on cytochrome P4501A (CYP1A) expression and activity in juvenile Atlantic salmon (Salmo salar L.) with regard to time and temperature was investigated. The time exposure study was performed at 8 ЊC, and the results show that PBO, although acutely functioning as a CYP1A inhibitor, is able to induce CYP1A expression in salmon liver. Both PBO and BNF give highest induction of CYP1A mRNA 48 h after intraperitoneal injection (five-and 14-fold, respectively). The mRNA levels induced by PBO and BNF were sustained during the 8 d of the experiment (four-and 11-fold, respectively). The CYP1A protein content measured by enzyme-linked immunosorbent assay demonstrated the highest induction by PBO 8 d after exposure (eightfold) and by BNF 4 d after exposure (11-fold). Activity of CYP1A measured by ethoxyresorufin-O-deethylase (EROD) demonstrated inhibition after PBO treatment the first 24 h after exposure, followed by threefold induction from 48 h and to the end of the experiment (8 d). -Naphthoflavone strongly induced EROD activity, with the highest levels occurring 4 d after treatment (56-fold) and 8 d after treatment (22-fold). In the temperature study, the results demonstrated temperature compensation, as salmon acclimated to 7ЊC for 3 weeks had a significantly higher EROD activity than those acclimated to 11 and 15ЊC. This was not reflected in significantly higher levels of reduced nicotinamide adenine dinucleotide phosphate cytochrome c reductase activity or CYP1A protein. The inductive properties of PBO and BNF on CYP1A expression was also demonstrated in primary cultures of salmon hepatocytes.