Accurate techniques monitoring the depth of anesthesia have been studied since the introduction of general anesthesia. Currently, however, no one technique has been proven uniformly reliable or accurate. Assessment of anesthetic depth still rests on clinically relevant signs in use more than a century, which are variable among different anesthetics and correlate imperfectly with anesthetic depth. The purposes of this study was to elicit the usefulness of the activity of LEC(lower esophageal contractility) as a criterion of anesthetic depth under the various halothane concentrations comparing with the other parameters like blood pressure, heart rate, frequency and amplitude of EEG, amplitude of EMG, and plasma epineprine and norepinephrine concentration. The materials were 56 ASA class I surgical patients, aged from 20 to 49, who had no specific past medical history or recent medication and divided into 4 groups composed of 7 male and 7 female patients. The baseline control measurements were undertaken 6 min after intubation without any anesthetics and the comparative measurements were undertaken 20 min after maintenance of anesthesia with continuous end-tidal halothane concentration of 0.4 Vol% (group I), 0.8 Vol% (group 2), 1.1 Vo1% (group 3), and 1.5 Vol%(group 4) respectively. The results were as follows; I) The changes of activities of LEC according to increasing end-tidal concentration of halothane. a. Response rates of spontaneous LEC were 100%(l4/14), 57%(8/14), 29%(4/14) respectively with significant decreases. b. Frequencies of spontaneous LEC were 0.8+/-0.4/min, 0.4+/-0.1/min 0.3+/-0.1/min respectively with significant decreases between group I and group 2, 3. Correlation coefficient(r) with halothane concentrations was -0.63. c. Amplitudes of spontaneous LEC were 37+/-ll, 31+/-16, 33+/-17 mmHg respectively without significant differences. The r was 0.1. d. Amplitudes of provoked LEC were 21+/-11, 13+/-11, 10+/-3, l0+/-3 mmHg respectively with significant decreases between group 1 and group 3, 4. The r was -0.44. e. LEC index were 74+/-21, 27+/-13, 17+/-9, 11+/-6 respectively with significant decreases between group l and group 2, 3, 4, and between group 2 and group 3, 4 The r was -0.74. 2) Comparisons with the correlation caefficients of other parameters according to halothane concentration. The correlation coefficient of Frequency of EEG with halothane concentration was -0.75 and that of LEC index was -0.74, and diastolic blood pressure -0.70, mean arterial pressure -0.64, amplitude of spontaneous LEC -0.63, systolic blood pressure -0.49, plasma epinephrine eoncentration -0.48, amplitude of EEG -0.46, amplitude of provoked LEC -0.44 in order. The absolute values of the correlation coefficients of other measurements were less than 0.3. Above findings suggested that frequency of spontaneous LEC and amplitude of provoked LEC decreased with increasing halothane concentration. It was also proven that LEC index had good correlation with halothane concentration and could be useful as a criteriori of anesthetic depth.