Previous studies showed that hydrocarbon induction of hepatic microsomal monooxygenase activity is attenuated in the teleost fish Fundulus heteroclitus acclimated to low temperature. The basis of that attenuation, and the effects of temperature on monooxygenase activity, were examined by analyzing liver cytochrome P4501A (CYP1A) mRNA, protein, and catalytic activity in control and beta-naphthoflavone (BNF)-treated F. heteroclitus acclimated to 6 or 16 degrees C. There were no temperature-related differences in total P450 content, NADPH-cytochrome c (P450) reductase activity, ethoxyresorufin O-deethylase (EROD) activity, or immunoquantified CYP1A content in hepatic microsomes of untreated fish. Fish acclimated to 16 degrees C and given a single intraperitoneal injection of BNF exhibited a rapid rise and fall in CYP1A mRNA content and an induction of EROD activity and CYP1A protein that was undiminished over 7 days. Similarly treated fish acclimated at 6 degrees C showed an increase in CYP1A mRNA content greater than that in 16 degrees C fish, but with no significant increase in EROD activity or CYP1A content over 7 days. Examined over a longer term, microsomal EROD activity was significantly induced by BNF in fish at both temperatures; activity peaked at 5-7 days in 16 degrees C fish, while in 6 degrees C fish the activity continued to rise slowly over 25 days. However, the greatest activity reached in 6 degrees C fish (0.68 nmol/min/mg) was less than half that seen in the warmer animals (1.46 nmol/min/mg). Immunodetectable CYP1A content showed the same trend as EROD activity, and the turnover number (nmol product formed/min/nmol CYP1A) for EROD activity was about the same in all groups, indicating that concentration of the catalyst alone could account for the different patterns of microsomal activity. CYP1A mRNA content was again induced to a similar degree by BNF in both the 6 and the 16 degrees C fish; the apparent half-life of the mRNA was substantially longer in cold-acclimated than in warm-acclimated BNF-treated fish. Comparing the levels of CYP1A mRNA and protein at the two acclimation temperatures following BNF treatment indicates that translational activity, rather than transcriptional activity, is the sensitive point in the effect of temperature on CYP1A induction in these fish.