BACKGROUND: This study was aimed to elucidate the effect of ketamine anesthesia on circulatory response to hemorrhage in rats by power spectral analysis of heart rate and blood pressure variability. METHODS: Nineteen male Sprague-Dawley rats weighing 290~475 g were divided into ketamine (100 mg/kg, im)-anesthetized(K, n=10) and conscious(C, n=9) groups. Hemorrhage was induced with a withdrawal pump from the femoral artery at 3 ml/kg/min for 5 min. Arterial pressure was measured with a pressure transducer connected to the contralateral femoral artery for 5 min before, during and after hemorrhage. The blood pressure signal digitized at 500 Hz through a data acquisition system was analyzed with fast Fourier transform algorithm to yield power spectra of sytolic(SP) and diastolic(DP) blood pressure and instantaneous heart rate(HR). Powers of very low frequency(VLF, 0.02~0.26 Hz), low frequency (LF, 0.26~0.75 Hz) and high frequency(HF, 0.75~5.00 Hz) band were expressed as percent of total power. RESULTS: Before hemorrhage blood pressure was higher in K(152.4+/-3.7/99.9+/-4.9 mmHg) than in C(143.3+/-5.7/95.5+/-4.1 mmHg) rats, but was changed by hemorrhage in both groups. Before hemorrhage HR in K and C rats were 361.4+/-17.5 and 363.4+/-18.5 beats/min . HR were significantly increased to 403.2+/-20.3 and 396.2+/-18.9 beats/min during and after hemorrhage in K rats, and increased to 409.1+/-20.9 beats/min during hemorrhage in C rats. Before hemorrhage total powers of blood pressure and HR variability were higher in K than in C rats. During hemorrhage, total powers of blood pressure and heart rate variability tended to increase in both groups. Before hemorrhage, percent powers of systolic pressure variability of HF and VLF were higher in K than in C rats and LF was lower in K than in C rats. During hemorrhage, K group showed no significant changes but C group showed significant changes. Before hemorrhage, percent powers of diastolic pressure variability of VLF was higher in Kthan in C rats, and HF and LF were lower in K than in C rats. During and after hemorrhage, K group showed no significant changes, but C group showed significant decrease in LF and increase in VLF. Before hemorrhage, percent powers of heart rate variability of K rats showed higher HF and VLF, and lower LF than C rats. During and after hemorrhage HF and VLF of both groups showed no significant changes except significant increase in VLF after hemorrhage in C rats, but LF of both groups showed significant decrease. CONCLUSIONS: It was concluded that autonomic activity, especially cardiac sympathetic activity, was increased in response to hemorrhage in K rats. Ketamine anesthesia stimulated overall autonomic activity, especially sympathetic activity and vasomotor tone. In C rats hormonal factor contributed to blood pressure and heart rate variability during hemorrhage.