BACKGROUND: This study was aimed to elucidate the effect of thiopental anesthesia on circulatory response to hemorrhage in rats by power spectral analysis of heart rate and blood pressure variability. METHODS: Sixteen male Sprague-Dawley rats weighing 350~475 g were divided into thiopental (50 mg/kg, ip)-anesthetized (T, n=10) and conscious (C, n=6) 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 lower in T (113 6/83 6 mmHg) than in C (157 4/101 2) rats, but was not changed by hemorrhage in both groups. Before Hemorrhage HR was lower in T (331 22 beats/min) than in C (378 27) rats. HR was significantly increased to 412 26 beats/min in C, but was not changed in T during hemorrhage. Total powers of Before Hemorrhage blood pressure and HR variability were lower in T than in C. During hemorrhage, total power of blood pressure variability tended to increase in both groups, and that of DP was significantly increased during hemorrhage in T. Total power of HR was significantly increased in C, but was not changed in T. T rats showed lower LF and higher HF power of blood pressure, and lower VLF and LF and higher HF power of HR than C rats. During hemorrhage, both groups showed no significant change in spectral distribution of HR power, except for a significant increase in LF of HR after hemorrhage in C. Spectral distribution of blood pressure power was not changed by hemorrhage in C. In T rats, VLF of DP was significantly increased and HF of DP was decreased during hemorrhage. CONCLUSIONS: It suggests that autonomic activity, especially cardiac sympathetic activity is increased in response to hemorrhage in C. Thiopental anesthesia depresses overall autonomic activity, especially sympathetic activity, and vasomotor tone. Hemorrhage under thiopental anesthesia induces depression of baroreceptor reflex activity, while increased levels of vasoconstrictor hormones help to maintain the blood pressure.