The purpose of this investigation was to evaluate the effect of moderate (non-hypertensive) levels of muscle tension on the cardiovascular responses to progressive lower body negative pressure (LBNP) in eight healthy male volunteers. Subjects were presented with progressive LBNP to -50 torr or the occurrence of vasovagal symptoms during three different levels of electromyographic activity in the lower limbs represented by the relaxed state, 5 and 10% maximal voluntary contraction. The same procedure was also performed at the same three levels of electromyographic activity in the arms with concomitant relaxation of the abdomen and lower extremities. In 75% of the subjects, pre-syncopal reactions were observed during the relaxed state while no pre-syncopal responses occurred during the elevated muscle tension levels. Both levels of muscle tension in the legs attenuated the LBNP-induced decrease in blood pressure (P less than 0.05). The effect of the 5% maximal voluntary contraction tension level appeared to be due to a compressive effect on the vascular tree, because similar levels of tension in the forearm had minimal effect on the blood pressure response to LBNP. In addition to a compressive effect, the 10% maximal voluntary contraction tension level appeared to induce a reflex stimulation of the heart as evidenced by an augmented heart rate response to LBNP and an increase in cardiac output. The muscle tension appeared to induce a mechanical compression of the vascular tree which was accompanied by somatopressor reflex responses, resulting in a maintenance of blood pressure that was primarily mediated by a maintained cardiac output.