The fractional synthesis rate of collagen (the percent of total ventricular collagen synthesized in one day) was measured in the hearts of normal dogs and those with pulmonary artery stenosis using a continuous 6-h intravenous infusion of [14C]proline. The fractional synthesis rate in normal ventricles was slow, 0.56%/day, and it increased eightfold to 4.8%/day in the hypertrophying right ventricle after 5 days. After 12 and 28 days the synthesis rate was still significantly greater than in the control left ventricle, being 2.6 and 1.3%/day, respectively. However, the synthesis rate of noncollagen protein was significantly greater than normal at 5 days only. The collagen content (expressed as a mass fraction) of the right ventricle decreased over the first 12 days of hypertrophy but by 28 days was restored to the normal right ventricular value of 9.6 mg/g wet wt tissue. The total amount of collagen in the hypertrophied ventricle calculated from the synthesis rates was in accord with that measured chemically. In normal dogs the collage content of the right ventricle was greater than that of the left, and the epicardium contributed substantially to the total collagen in the ventricular walls.