Abstract: Every year more than 100 million people travel through mountains. Many of these travelers are middle aged and even elderly in whom there is minimal cardiovascular research. In this study we evaluated sympathico-vagal tone before and after exposure to middle altitudes. 13 subjects, 7 women and 6 men, ages 45 to 72 were evaluated before and after exposure to middle altitudes. Evaluations consisted of cardiac loading during echocardiography as well as calculating the vegetative tone by using the Kerdo index. Vegetative tone was analyzed using the Fisher’s maximum-likelihood estimation, after which the integral under the line of best fit was obtained with limits defined as the shortest loading test in seconds when compared to baseline. Time to reach 85% maximally predicted cardiac rate increased for almost all subjects after exposure to middle altitudes. For the comparison of the vegetative control the integral of the fitting exponentials over time has been utilized. As demonstrated by an increase in the Kerdo index, 10 out of 13 of the subjects showed decrease followed by increase of the sympathetic responce as they acclimatized. Two of the three who did not demonstrate this shift has significant cardiovascular disorders which was discovered prior to exposure. We have demonstrated decrease followed by increase of the sympathetic response during proper acclimatization. We demonstrated this with the vegetative Kerdo index in 10 healthy subjects. Furthermore the majority of the subjects with underlying cardiac pathology failed to demonstrate this shift.