The distribution and function of AII receptor subtypes was evaluated in different preparations of rat hearts. Autoradiographic experiments and binding experiments on isolated membranes showed a large expression of [125I]Sar1,Ile8-AII binding sites in the atria of neonatal Wistar Kyoto rats which were predominantly of the AT2 subtype. Atrial and ventricular cells, isolated from neonatal rat hearts and maintained for 3 days in culture demonstrated primarily AT1 binding sites. Stimulation of cultured atrial cells with AII resulted in an increase in inositol phosphate turnover and in intracellular calcium. The latter action was completely abolished by Losartan. Finally, in atria isolated from 2-month-old rats, AII produced a 17-19% increase in contractile force that was completely abolished by Losartan but not by PD 123319, thus indicating the presence of functional AT1 receptors.