An increase in aortic pressure from 60 to 120 mmHg accelerated ribosomal protein synthesis in rat hearts during 1 or 2 h of labeling with 0.4 mM [3H]phenylalanine. When hearts were perfused with buffer that contained 20 mM glucose and normal plasma concentrations of 19 other amino acids without added insulin, ribosomal protein synthesis relative to the rate of total protein synthesis increased from approximately 0.22 to 0.36 and 0.30 as aortic pressure was raised from 60 to 120 mmHg during 1 or 2 h of labeling, respectively. With the addition of insulin, the relative rate of ribosomal protein synthesis averaged 0.33 at an aortic pressure of 60 mmHg and increased to 0.42 when aortic pressure was raised to 120 mmHg. These results indicate that elevation of aortic pressure has a preferential effect on synthesis of new ribosomes. This response appears to be an early and physiologically significant event in cardiac hypertrophy.