We demonstrate the establishment of transgenic mice, where the expression of the green fluorescent protein (GFP) is under control of the human cardiac alpha-actin promoter. These mice display cardiac specific GFP expression already during early embryonic development, Prominent GFP fluorescence was observed at the earliest stage of the murine heart anlage (E8), Cardiomyocytes of different developmental stages proved GFP positive, but the intensity varied between cells. We further show that contractions of single GFP positive cardiomyocytes can be monitored within the intact embryo. At later stages of embryonic development, the skeletal musculature was also GFP positive, in Line with the known expression pattern of cardiac alpha-actin. The tissue specific labeling of organs is a powerful new tool for embryological as well as functional investigations in vivo, (C) 1998 Federation of European Biochemical Societies.