Since the neurohypophyseal hormone 8-arginine vasopressin (AVP) is involved in cardiovascular tissue hypertrophy and myocyte differentiation, it is possible that local AVP plays a role in heart maturation. AVP-specific RIA, RT-PCR, and immunoblot measurement of AVP receptors (VR) were used to investigate heart tissues from newborn and adult rats. To test AVP's role in differentiation and specialization into ventricle-like cardiomyocytes, we studied GFP-P19Cl6 stem cells, which express green fluorescence protein (GFP) reporter under transcriptional control of the myosin light chain-2v promoter. VR₁ transcripts and proteins were higher in adult than in newborn rat hearts. In contrast, VR₂ increased from postnatal day 1 to 5 and was barely detected in the adult rat heart. In cardiomyocytes expressing troponin C, immunofluorescence revealed VR₂ and VR₁. Intracellular cAMP increased 6.5- and 8.9-fold in response to the selective VR₂ agonist 1-desamino-8-d-AVP (DDAVP) after 1 and 24 h, respectively. Cardiac AVP was high in 1- and 5-day-old (330 ± 26 and 276 ± 53 pg/mg protein, respectively) but low in 66-day-old (98 ± 15 pg/mg protein) rats. AVP immunostaining was detected in the tunica adventitia and endothelium of the coronary vessels. The possible role of AVP in cardiomyogenesis was indicated by DDAVP-AVP-dependent differentiation of GFP-P19Cl6 stem cells into contracting cells displaying GATA-4, a cardiac-specific marker, and ventricle-specific myosin light chain. Together, it is suggested that the AVP system is implicated in postnatal cardiac maturation.