BACKGROUND Recently, the knowledge regarding all types of shock has increased. In the critical period, the maintenance of perfusion to important organs and of autonomic cardiovascular control is a clue to improve outcome. The current investigation studied the effects of endotoxin shock on the heart rate variability (HRV), which might be an early marker of multiple organ dysfunction syntion. METHODS CL 57 BL/6 J mice (30-32 g) were implanted with intra-abdominal telemetry transmitters and data collection began 7 days after surgery and continued for 5 hours after the treatment. In endotoxin group, 3 mg x kg(-1) of E. coli lipopylysaccharide (0127 : B 8) was administered intraperitoneally and the same dose of saline was administered in control group. From the beat-by-beat values, heart rate (HR), standard deviation and coefficients of variation (RMSSD) were calculated. Integrated values of power spectral density were derived by summing depending on the frequency ranges, which were chosen with respect to the results of the previous investigation as follows: very low frequency (VLF) < 0.15 Hz; low frequency (LF), 0.15-1.5 Hz; and high frequency (HF), 1.5-5.0 Hz. RESULTS HR showed no difference between the groups. RMSSD, Total Power, VLF, LF and HF increased in control group throughout the experiments, but, decreased in endotoxin group even 5 hr after the treatment. CONCLUSIONS There has been scarce information on mice HRV, but we have demonstrated that endotoxin administration significantly decreases RMSSD and spectral density at low and high frequencies.