A method of causing cardiomyocyte growth and/or differentiation, the method comprising exposing a cardiomyocyte to neuregulin (NRG) thereby activating the MAP kinase pathway in the cardiomyocyte and causing growth and/or differentiation of the cardiomyocyte. Use of neuregulin, neuregulin polypeptide, neuregulin derivatives, or compounds which mimic the activities of neuregulins in the treatment or management of heart disease and heart failure in a mammal.