Aims The aim was to evaluate the changes in approximate entropy (ApEn) calculated for hemodynamic traces before (0 mmHg) lower body negative pressure (LBNP) and during the first part of the test (-15 mm Hg) in 13 subjects with high (HT) and 11 of low (LT) tolerance to LBNP. Material and Methods 24 healthy, untrained male subjects (age:20.8±0.9 yrs, height: 176.9±4.3 cm, body mass: 74.2±7.1 kg) were submitted to graded LBNP (-15, -30 and -50 mmHg) lasting 10 minutes for each load. The length of RR interval (RR), stroke volume (SV) and ejection time (ET) were determined basing on ECG and first derivative (dz/dt) of impedance cardiography (ICG) traces. Analysis were carried out for unprocessed dz/dt waveform and for hemodynamic parameters time series (RR, SV, ET) by determining the fractal dimension using the Higuchi fractal algorithm. Results ApEn of dz/dt raw signal in no load (0 mmHg) period was 0.23 (range:0.18-033) and 0.24 (0.21-0.29) (NS), in LT and HT, respectively. For the load of -15mmHg the ApEn was 0.25 (0.16-0.40) and 0.25 (0.18-0.34 (NS), respectively. For SV the ApEn at 0 mmHg was 1.15 (range:0.87-1.30) and 1.13 (0.87-1.30) (NS), in LT and HT, respectively. During the -15mmHg period the results were 1.36 (1.27-1.46) (LT) and 1.31 (1.18-1.49) (HT). At rest data for ET were: 1.07 (0.68-1.35) (LT) and 1.05 (0.81-1.24) (HT), whereas in -15mmHg load: 1.21 (0.98-1.43) (LT) and 1.25 (0.98-1.34) (HT). At rest data for RR were:1.06 (0.76-1.19) (LT) and 1.05 (0.99-1.18) (HT), whereas in -15mmHg load: 1.14 (0.82-1.36) (LT) and 1.15 (0.91-1.40) (HT). Conclusions No differences in ApEn were found between LT and HT for raw dz/dt traces and RR, SV, ET time series. ApEn for RR, SV and ET were similar in both periods. ApEn seems to be not suitable for predicting the outcome of LBNP test.