Chronic pressure overload is known to increase cardiac mass and expression levels of both atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) mRNAs. Although mechanical stretching of cardiac myocytes could cause these changes, humoral factor(s) secondary to pressure overload may also be involved. To dissociate humoral effects from the effects of mechanical loading on cardiac hypertrophic responses, we examined expression of ANP and BNP at both mRNA and protein levels and proportions of myosin isoforms in transplanted cervical hearts that were mechanically unloaded under conditions with or without hypertension by aortic coarctation. Seven days after transplantation, cardiac atrophy that usually occurs in transplanted hearts without hypertension by coarctation was prevented in the transplanted hearts with hypertension by coarctation. The levels of expression of ANP and BNP mRNAs were increased in the transplanted hearts with relative to those without hypertension by coarctation. The plasma level of angiotensin II was higher in rats with than without hypertension by coarctation. Plasma endothelin-1 levels were not significantly different between the two groups. In addition, levels of expression of ANP and BNP mRNAs were increased in the transplanted hearts without hypertension relative to those in the in situ hearts. The proportion of the V3 myosin isoform was also increased in the transplanted hearts without hypertension relative to the in situ hearts. These results indicate that humoral factor(s) secondary to the pressure overload produced by aortic coarctation enhanced the cardiac hypertrophic response and elevated the levels of mRNAs encoding these embryonic markers. Moreover, our findings regarding ANP and BNP expression in the transplanted hearts provide additional evidence that the fetal genes are reexpressed during the process of cardiac atrophy as well as in cardiac hypertrophy.