SummaryThe right ventricular (RV) and left ventricular (LV) free wall weights, ornithine decarboxylase (ODC) specific activity and polyamine content were determined in fetal, 1–, 2–, 7–, 14–, and 21–day-old rabbit hearts. There was a significant increase in the LV free wall weight and a decrease in RV free wall weight between 1–7 days. By day 7 the LV/RV mass ratio doubled and reached a ratio comparable to that seen in adult rabbit hearts. The rate of change in the LV and RV free wall weights were comparable after day 7.There was a significant increase in LV ODC specific activity and a decrease in RV ODC specific activity after birth. The LV ODC specific activity was significantly greater than RV ODC specific activity in both 1 and 2 day old rabbit hearts whereas they were not significantly different in 7 and 14 day old rabbit hearts.The molar content (nmoles/mg wet weight or nmoles/mg protein) of putrescine decreased approximately 3-fold fter birth in the RV but increased approximately 2-fold in the LV. The molar content of spermidine and spermine was transiently increased after birth (1 day old) in both RV and LV with approximately a 2-fold increase in spermidine and a 2.5-fold (LV) and 4-fold (RV) increase in spermine content. The increase in LV putrescine content after birth was due, at least in part, to the observed increase in ODC specific activity in the LV free wall. The putrescine/spermidine ratio increased in the LV and decreased in the RV immediately after birth up to day 7. As opposed to the shortlived increase in spermidine and spermine observed in both the RV and the LV free wall, increased ODC specific activity and putrescine accumulation uniquely characterized the preferential growth of the LV between day 1 and 7.Speculation The postnatally developing heart affords an ideal model for studying the relationship between pressure loading and adaptive myocardial growth because paired comparisons of left and right ventricular parameters are possible. Age-related changes in left ventricular (LV) growth, ornithine decarboxylase (ODC) specific activity and putrescine accumulation occur immediately after birth and were clearly different from the right ventricle. This increase in LV ODC activity and putrescine synthesis may be secondary to the alterations in hemodynamic loading of the LV in the newborn. If cellular hyperplasia accounts for this preferential growth of the LV in the developing rabbit heart, as has been reported to be the case in the developing rat heart, it is speculated that polyamine metabolism may play a role in myocardial cellular hyperplasia.