Over a 16-day period post-hypophysectomy both atrial natriuretic factor (ANF) mRNA and peptide levels were quite differently regulated in right and left ventricles. In the right ventricle ANF mRNA levels showed a large increase 2 days post-hypophysectomy, and returned to control levels by day 8. In the left ventricle mRNA levels doubled by day 2, and continued to rise to reach plateau levels (approximately 3-fold control) 8-16 days post-hypophysectomy. Tissue levels of ANF peptide were also measured by radioimmunoassay over the same time period. Right ventricular ANF levels rose 10-fold 2 days post-hypophysectomy, and returned to 3 times control levels by 8 days, mirroring the changes in mRNA. In the left ventricle, however, ANF peptide levels did not change significantly from intact values over the entire 16-day period following hypophysectomy despite the progressive increase in ANF mRNA levels in this tissue, suggesting that the dichotomy between mRNA and peptide levels in the left ventricle reflects an increased rate of peptide secretion.