Objective: To evaluate and compare the autonomic heart rate (HR) modulation, under resting conditions in relation to body posture, in sedentary young adults. Methods: Twenty young healthy and sedentary men aged 22.6 ± 2.5 years participated in the study. The HR and R-R intervals (in ms) of the electrocardiogram (EKG) were obtained in real time using the modified DII derivation, with the volunteers at rest in the supine and sitting positions, for 15 minutes. The R-R data were analyzed in the time domain, by means of the RMSSID, RMSM and pNN50 (%) indices; and in the frequency domain, by means of spectral analysis and fast Fourier transforms (FFT), using low frequency (LF) and high frequency (HF) bands expressed as normalized units and as the LF/HF ratio. The statistical analysis consisted of the Spearman test for correlation analyses and the Wilcoxon test for paired samples, with significance of α= 5%. Results: In the time domain, the RMSSD and pNN50 indices demonstrated statistically significant differences between the supine and sitting positions (p< 0.05). the RMSM index did not showed any statistically significant difference between the positions (p>0.05). In the frequency domain, the LF and HF bands and the LF/HF ratio demonstrated statistically significant differences between the supine and sitting positions (p<0.05). Conclusions: The results demonstrated that by changing the posture, autonomic adjustments were produced to the parasympathetic and sympathetic nervous systems with regard to HR control. This can be attributed to the integrity of the neurocardiac system.121712Glick, G., Braunwald, E., Relative role of the sympathetic and parasympathetic nervous systems in the reflex control of heart rate (1965) Cir Res, 16, pp. 363-375Leon, D.F., Shaver, L.A., Leonard, J.J., Reflex heart rate control in man (1970) Am heart J, 80, pp. 729-739Mitchell, J.J.B., Wolffe memorial lecture. Neural control of the circulation during exercise (1990) Med Sci Sports Exerc, 22 (2), pp. 141-154Longo, A., Ferreira, D., Correia, M.J., Variabilidade da freqüência cardíaca. (1995) Rev Port Cardiol, 14 (3), pp. 241-262Antila, K., Quantitative characterization of heart rate during exercise (1979) Scand J Clin Lab Invest, 80, pp. 153-155American College of Cardiology/American Heart Association-Clinical competence statement j stress testing. J Am Coll Cardiol. 2000;(36):1441-53Silva, E., Catai, A.M., Trevelin, L.C., Guimarães, J.O., Silva Jr, L.P., Silva, L.M.P., Desing of computerized system to evaluate the cardiac function during dynamic exercise (1994) Annals of the Word Congress on Medical Physics and Biomedical Engineering, 1, p. 409Task Force of European Society of Cardiology of the North American Society of Pacing Electrophysiology. Heart rate variability. Standars of mensurement, physiological interpretation and clinical use. Circulation. 1996;93:1043-65Gallo Jr, L., Maciel, B.C., Marin-Neto, J.A., Martins, L.E.B., Lima-Filho, E.C., Golfetti, R., Control of heart rate during exercise in health and disease (1995) Braz J Med Biol Res, 28 (11-12), pp. 1179-1184Marães, V.R.F.S., Santos, M.D.B., Catai, A.M., Moraes, F.R., Oliveira, L., Gallo Jr, L., Modulação do sistema nervoso autonômico na resposta da freqüência cardiaca em repouso e à manobra de valsalva com o incremento da idade. (2004) Rev Bras Fisioter, 8 (2), pp. 97-103Sakabe, D.I., Catai, A.M., Neves, V.F.C., Oliveira, L., Silva de, S.M., Azevedo, G.B., Análise da modulação do coração durante condições de repouso em homens de meia-idadeie mulheres pós-menopausa. (2004) Rev Bras Fisioter, 8 (1), pp. 89-95Acharya, U.R., Kannathal, N., Hua, L.M., Yi, W., Study of heart rate variability signals at sitting and lying postures (2004) J Bodywork Mov Theraphies, pp. 2-8Lindquist, A., Noninvasive methods to study autonomic nervous control of circulation (1990) Acta Physiol Scand Suppl, 588, pp. 1-107Siebert, J., Drabik, P., Lango, R., Szyndler, K., Stroke volume variability and heart rate power spectrum in relation to posture changes in hearty subjects (2004) Med Sci Minit, 10 (2), pp. MT31-MT37Malik, M., Camm, A.J., Heart rate variability (1990) Clin Cardiol, 13 (8), pp. 570-576Maia, I.G., (1967) Eletrofisiologia clínica e intervencionista das arritmias cardíacas, , Rio de Janeiro: Revinte;Paschoal, M.A., Volanti, V.M., Pires, C.S., Fernandes, C.S., Variabilidade da freqüência cardíaca em diferentes faixas etárias. (2006) Rev Bras Fisioter, 10 (4), pp. 413-419Mello, R.C., Santos, M.D.B., Silva, E., Quitério, R.J., Moreno, M.A., Reis, M.S., Effects of age and physical activity on the autonomic control of heart rate in healthy men (2005) Braz J Med Biol Res, 38, pp. 1331-1338Bigger, J.T., Albrecht, P., Steinman, R.C., Rolnitzky, L.M., Fleiss, J.L., Cohen, R.J., Comparison of time and frequency domain-based measures of cardiac parasympathetic activity in holter recordings after myocardial infarction (1989) Am J Cardiol, 64, pp. 536-538