Analyses on the cellular level are indispensable to expand our understanding of complex tissues like the mammalian heart. Single nucleus sequencing (snRNA-seq) allows for the exploration of cellular composition and cell features without major hurdles of single cell sequencing. We used snRNA-seq to investigate for the first time an entire adult mammalian heart. An additional RNA velocity analysis allowed to study transcription kinetics and was utilized to visualize the transitions between mature and nascent cellular states of the cell types. We identified subgroups of cardiomyocytes with distinct marker profiles.