This study on dogs determined whether the requirement for enflurane anesthesia was different pre- versus postcardiopulmonary bypass (CPB). Male mongrel dogs (n = 16) were anesthetized with enflurane in oxygen. Tracheal intubation was performed, monitors placed, and end-tidal enflurane concentration measured via a Puritan-Bennett Anesthesia Agent Monitor. MAC was determined by the tail-clamp method. CPB was then initiated using aortoatrial (n = 6, group 1) or femoral artery-vein (n = 4, group 2) cannulation or none (n = 6, group 3, control). CPB was maintained for 1 h using a bubble oxygenator, a crystalloid prime, and flows of approximately 70-80 ml/kg with a mean systemic pressure maintained between 50-70 mmHg. Following separation from CPB, MAC was again determined. The reduction in enflurane MAC following CPB was 30.1 +/- 21.5% (mean +/- SD; P less than 0.05 vs. pre-CPB) in group 1 but there was a wide range of reduction produced (3.8-58.8%). The degree of MAC reduction (19.8 +/- 8.6%; P less than 0.05 vs. pre-CPB) produced by CPB in group 2 was much less variable in degree (range 13.0-32.4%) but did not differ from group 1. Although pre- versus post-CPB mean systemic pressure fell from 83 +/- 13 to 69 +/- 15 mmHg (P less than 0.05), this is above the level likely to produce a reduction in MAC. No other significant hemodynamic changes were observed. Temperature pre- versus post-CPB was not different. The degree of hemodilution and acid-base disturbances are unlikely to be the explanation.(ABSTRACT TRUNCATED AT 250 WORDS)