Two of the neuregulins (NRG1 and NRG2) and their receptors (erbB2 and erbB4) are essential for normal cardiac development and can mediate hypertrophic growth and enhance survival of embryonic, postnatal, and adult rat ventricular myocytes. The expression of erbB4, the predominant NRG receptor in postnatal rat ventricular muscle, declines after midembryogenesis, and its expression is limited to cardiac myocytes. A full-length erbB4 rat cDNA isolated from neonatal ventricular muscle was found to be highly homologous to human erbB4 and contained a caveolin binding motif within the cytoplasmic kinase domain. Using the complementary techniques of detergent-free density-gradient ultracentrifugation of myocyte lysates and coimmunoprecipitation of erbB4 and caveolin-3, the caveolin isoform expressed in cardiac myocytes, erbB4 could be localized (using both approaches) to caveolar microdomains. Moreover, addition of a soluble NRG1, recombinant human glial growth factor 2, resulted in rapid (2-minute) translocation of erbB4 out of caveolar microdomain in cardiac myocytes. Thus, erbB4 is dynamically targeted to caveolar microdomains within cardiac myocytes. Its rapid translocation after NRG1 binding may contribute to receptor desensitization in the continuous presence of ligand.