The first hours of extracorporeal life support (ECLS) are commonly marked by new hemodynamic instability without a known etiology. We measured hormone and catecholamine concentrations in six ECLS primed circuits immediately before joining the patient's circulation to assess a potential role of these agents in this condition. The following hormones were significantly below the lower end of the normal range for the first week of life (data are presented as mean +/- SEM): cortisol 1.95 +/- 0.15 microg/dl (p < 0.001), aldosterone 3.73 +/- 0.74 ng/dl (p < 0.05), free thyroxine 1.2 +/- 0.1 ng/dl (p < 0.05), free triiodothyronine 0.53 +/- 0.03 pg/ml (p < 0.001), thyroid stimulating hormone 0.31 +/- 0.05 microU/ml (p < 0.001), growth hormone (GH) 0.09 +/- 0.01 ng/ml (p < 0.001), estradiol 38.3 +/- 3.72 pg/ml (p < 0.001), IGF-BP1 0.95 +/- 0.1 ng/ml (p < 0.001), glucagon 26 +/- 1.2 pg/ml (p < 0.001), epinephrine 17.3 +/- 3.7 pg/ml (p < 0.001), and norepinephrine 127 +/- 27 pg/ml (p < 0.05). No dopamine was detected. Normal hormone concentrations included IGF-I, IGF-BP3, insulin, parathyroid hormone, leptin, and testosterone. Critically low concentrations of cortisol, thyroid hormones, GH, IGF-BP1, glucagon and catecholamines were measured in the ECLS circuit even though it was primed with fresh frozen plasma. These concentrations may cause significant and precipitous dilutional reductions in the patient's circulating levels immediately after connection to the ECLS circuit and hence contribute to hemodynamic instability.