Over a 12-month period, 1,743 patients were retrospectively evaluated for anaphylactic/anaphylactoid reactions during cardiac surgery. Reactions to protamine, vancomycin, blood, and metocurine were observed in eight patients (0.46%). Baseline to reaction mean arterial pressures decreased from 81 +/- 9 mmHg to 50 +/- 7 mmHg (mean +/- SD; p less than 0.001), cardiac output increased from 4.6 +/- 0.6 L/min to 6.5 +/- 1.2 L/min (p less than 0.005), stroke volume increased from 49 +/- 11 to 83 +/- 22 ml/beat (p less than 0.02), and systemic vascular resistance decreased from 1.294 +/- 137 to 563 +/- 127 dyne/sec/cm-5 (p less than 0.001). Two patients developed pulmonary artery hypertension, while only one patient developed bronchospasm. Initial hypotension during anaphylactic/anaphylactoid reactions is due to decreased systemic vascular resistance, not myocardial depression.