Two-dimensional echocardiography was used in 15 normal volunteers to assess left (LV) and right ventricular (RV) responses, as well as their interdependence, during the Valsalva maneuver. During the strain phase, LV and RV areas decreased progressively, the RV area decreasing more than the LV area. Immediately after strain release, the RV end-diastolic area increased suddenly and dramatically to 143.3% +/- 9.4% of its baseline value, whereas the LV end-diastolic area decreased further. This transiently overloaded right ventricle and associated septal shift changed LV shape and further reduced the LV cross-sectional area. Thus the resulting momentary drop in the stroke LV area may contribute, along with pulmonary blood pooling, to the abrupt systemic blood pressure drop characteristic of phase III seen in normal subjects. Real-time imaging with echocardiography during respiratory maneuvers is feasible for clinical use. Its application in patients with congestive heart failure might bring further understanding of LV and RV interrelationships in the failing heart.