M-mode contrast echocardiography with peripheral venous injections was performed in 73 patients with interatrial communications: 48 with atrial septal defect, 19 with a patent foramen ovale, and six with complex congenital heart disease in addition to their atrial septal defect. During resting conditions a cross-over of echo-contrast from the right to the left atrium indicating a right to left shunt was observed in 41/48 (= 85%) of the atrial septal defect cases, including all with and 82% of those without Eisenmenger's reaction, in 7/19 (= 37%) with patent foramen ovale and in 83% of all examined patients. The echocardiographically detectable right to left shunt could often be increased or provoked by repeating injections during the Valsalva maneuver. Positive contrast studies either at rest or with Valsalva provocation were observed in 94% of the atrial septal defect group and 63% of the patients with patent foramen ovale. Two-dimensional contrast echocardiography was somewhat less sensitive in detecting right to left shunts, but allowed the demonstration of the left to right shunt in 79% of the patients with uncomplicated atrial septal defect. The left to right shunt was seen as a stream of contrast free left atrial blood entering the contrast filled right heart through the defect and causing a "wash out" phenomenon. When all available techniques were applied, either a right to left shunt (rest and/or Valsalva maneuver) or a left to right shunt was demonstrable in all cases of atrial septal defects.(ABSTRACT TRUNCATED AT 250 WORDS)