The rate-limiting enzyme in the biosynthetic pathway of catecholamines is tyrosine hydroxylase (TH), the activity of which is dependent on molecular oxygen. Zebrafish (Danio rerio) possess two non-allelic TH coding genes, TH1 and TH2. A principal goal of the present study was to determine if the expression of these genes is sensitive to environmental hypoxia. Additionally, we sought to determine if catecholamine content of larvae was changed by environmental hypoxia, and whether the hearts of hypoxic larvae were equally responsive to exogenous catecholamine (norepinephrine) exposure. After 2 days of exposure to hypoxia [5-7 days post-fertilization (dpf); PO(2) = 30 Torr] TH2 mRNA expression was significantly lower and dopamine β hydroxylase (DβH) mRNA was significantly higher in whole larvae. Whole body catecholamine levels were unchanged until after 4 days of hypoxic exposure (5-9 dpf), at which time there was a significant increase in epinephrine and norepinephrine contents. Norepinephrine content was significantly elevated in the hearts of adult fish after 2 and 4 days of hypoxic exposure, and TH1 mRNA expression was increased in the kidney of both groups. After 2 or 4 days of exposure to hypoxia, larvae displayed significantly lower heart rates than normoxic fish. However, application of exogenous norepinephrine caused similar increases in heart rate in both groups. Overall, it is concluded that the mRNA expression of TH1 and TH2 is differentially affected by hypoxia exposure in larvae and adults. Also, catecholamine biosynthesis appears to be activated by 2 dpf and although whole body catecholamine levels increase during hypoxia (possibly promoting downregulation of cardiac β-adrenergic receptors), there is no accompanying decrease in the response of the heart to adrenergic stimulation.