Studies using the head-up tilt test (HUT) suggest that a reflex increase in sympathetic activity resulting in vigorous myocardial contractions precedes neurally-mediated syncope (NMS). The aim of this study was to evaluate heart contractility changes during positive HUT. Ten patients with recurrent NMS and positive HUT were investigated. Before HUT we temporarily placed a standard right ventricular pacing electrode incorporating in its tip a recently developed microaccelerometer (Sorin Biomedica, Italy) that measures the peak endocardial acceleration (PEA) during the isovolumetric phase as an index of heart contractility. PEA potential amplitude, heart rate and mean blood pressure were continuously studied during HUT. Syncope occurred 16.7 +/- 10.3 min after 60 degrees tilt, either at baseline (8 patients) or after sublingual nitrate administration (2 patients). PEA value was stable at 0.62 +/- 0.34 (1G = 9.8 m/sec2) during the supine phase. It slightly increased to 0.72 +/- 0.44 G (p = NS) during the first minutes of 60 degrees tilt and then remained unchanged until a further increase of 71 +/- 79% (range 10 to 266%) as compared to tilt value (p = 0.004) at 2.8 +/- 2.4 min (range 0.25 to 6.5 min) before the syncope in 9 patients. The latter increase was not observed in the patient with dilated cardiomyopathy. In conclusion, a significant increase in heart contractility was observed in 9 patients in the minutes preceding HUT-induced NMS. These changes might be used for driving a rate adaptive pacemaker when cardiac pacing is indicated to prevent NMS.