OBJECTIVE To evaluate the effects of remifentanil on isoflurane minimum alveolar concentration (ISO(MAC)) in dogs. METHODS 6 adult mixed-breed dogs. METHODS Dogs were anesthetized with isoflurane on 2 occasions. During the first set of experiments, ISO(MAC) was determined before remifentanil infusion (baseline), during constant rate infusion (CRI) of remifentanil (0.15, 0.30, 0.60, and 0.90 microg/kg/min), and 80 minutes after remifentanil infusion. After a 1-week washout period, dogs received a CRI of remifentanil (0.15 microg/kg/min) and ISO(MAC) was redetermined 2, 4, and 6 hours after commencing the infusion. RESULTS Mean +/- SD baseline ISO(MAC) was 1.24 +/- 0.18%. Remifentanil infusion (0.15, 0.30, 0.60, and 0.90 microg/kg/min) decreased ISO(MAC) by 43 +/- 10%, 59 +/- 10%, 66 +/- 9%, and 71 +/- 9%, respectively. The ISO(MAC) values determined during the 0.30, 0.60, and 0.90 microg/kg/min infusion rates did not differ from each other, but these values were significantly lower, compared with the 0.15 microg/kg/min infusion rate. The ISO(MAC) recorded after remifentanil infusion (1.09 +/- 0.18%) did not differ from baseline ISO(MAC). There was no change in ISO(MAC) throughout the 6-hour period of a CRI of remifentanil. CONCLUSIONS Remifentanil decreased ISO(MAC) in a dose-related fashion; the reduction in ISO(MAC) was stable over the course of a prolonged CRI (6 hours). A dose of 0.30 microg of remifentanil/kg/min resulted in nearly maximal isoflurane-sparing effect in dogs; a ceiling effect was observed at higher infusion rates.