Experiments were conducted to determine the effect of surgical preparation and infusion of plasma and/or saline on circulating immunoreactive endothelin (irET) concentrations. Male Sprague-Dawley rats anesthetized with Inactin were placed on a surgical heating table for about 2 h before collecting blood via puncture of the abdominal aorta. The plasma concentration of irET was 8.1 ± 0.9 pg/ml in rats that did not undergo surgical preparation or receive any further treatment. Standard surgical preparation as for renal clearance experiments (catheters in the trachea, femoral artery, femoral vein, and bladder) resulted in a dramatic rise in irET to 17.7 ± 3.0 pg/ml (p < 0.05). Infusion of plasma (5 ml/kg over 20 min), but not saline, resulted in an additional increase to 32.6 • 3.5 pg/ml (p < 0.05). These results clearly demonstrate that standard whole-animal techniques such as surgery and plasma infusion produce significant changes in circulating endothelin which may affect a variety of physiological and pharmacological mechanisms being investigated in the anesthetized rat. © 1993, Informa UK Ltd. All rights reserved.