The anesthetic potency of a new fluorinated volatile anesthetic, I-653, was tested preliminary to testing its toxic and metabolic characteristics in rats. The minimum alveolar concentration (MAC) of I-653 at 37.9 +/- 0.2 degrees C (mean +/- SD) in eight rats was 5.72 +/- 0.40%. In five rats, we reduced the rectal temperature to 28 degrees C in two steps of 5 degrees C. MAC decreased by 0.238 +/- 0.036% I-653 per degree centigrade decrease in temperature. That is, MAC decreased 42% as temperature decreased 10 degrees C. We restored the temperature to 38 degrees C and redetermined MAC. This value (5.66 +/- 0.75%) did not differ from the average MAC obtained before hypothermia (5.84 +/- 0.36%).