Chronic supraventricular tachycardia (SVT) causes left ventricular (LV) dYsfunction and dilatation. Termination of SVT appears to improve symptoms of congestive heart failure. However, the structural events that occur during development and regression of SVT-induced cardiomyopathy are unknown. Accordingly, LV function (simultaneous echocardiogram-catheterization) and collagen content and distribution were measured in pigs (23-25 kg) assigned to three groups: 1) rapid atrial pacing (240 beats/min) for 3 wk (SVT, n = 10); 2) SVT for 3 wk, followed by deactivation of the pacemaker and a 4-wk recovery period (PST, n = 9); and 3) sham-operated controls (CON, n = 10). LV fractional shortening was 30 +/- 2% in CON, fell to 13 +/- 2% with SVT (P less than 0.05), and returned to CON values with PST (31 +/- 2%). SVT resulted in significantly increased LV end-diastolic dimension compared with CON (4.9 +/- 0.3 vs. 3.5 +/- 0.2 cm, P less than 0.05) and no change in LV wt/body wt (2.7 +/- 0.2 vs. 2.6 +/- 0.2 g/kg, P = 0.85). Termination of SVT (PST) resulted in development of hypertrophy, LV mass increased to 3.50 +/- 0.3 g/kg (P less than 0.05 vs. CON). With the use of pressure-dimension-thickness relations during diastole, the regional chamber stiffness constant (Kc) was computed. Kc was unchanged by SVT compared with CON (5.3 +/- 1.4 vs. 3.7 +/- 0.5, P greater than 0.35) but increased with PST (7.4 +/- 0.6, P less than 0.05). LV hydroxyproline content significantly fell with SVT compared with CON (2.24 +/- 0.58 vs. 2.68 +/- 0.45 mg/g dry wt, P less than 0.05, respectively) and significantly increased with PST (3.68 +/- 0.85 mg/g dry wt, P less than 0.05). With the use of transmission electron microscopy, collagen fibril diameter was reduced with SVT compared with CON (1.45 +/- 0.5 vs. 1.7 +/- 0.5 microns, P less than 0.05) and increased with PST (3.3 +/- 1.4 microns, P less than 0.05). Scanning electron microscopy revealed disruption of collagen struts between adjacent SVT myocytes and a thickened collagen weave with PST. Thus chronic SVT resulted in systolic and diastolic dysfunction and reduced collagen support of adjoining myocytes. Early recovery from SVT was associated with LV hypertrophy, increased collagen, and increased LV stiffness.