The expression of genes crucial for different cardiac functions changes during ontogenic development and during hypertrophy due to hemodynamic overload. To determine if these changes are transcriptional or post-transcriptional, we describe here an optimized method to analyze in vitro the transcriptional activity of nuclei isolated from control and hemodynamically overloaded hearts of 23-day-old rats. Similar results were obtained with both types of hearts: (1) 3 to 6 million nuclei were isolated per gram of ventricular tissue; (2) nuclear incorporation of UTP32P increases with time, plateaus after about 10 to 20 min. and attains an average of 0.17 cpm per nucleus; (3) alpha-amanitin decreases total incorporation by 56%; (4) the sizes of the majority of transcribed RNA are between 0.2 and 4 kb, although a significant fraction of high molecular weight transcripts are also seen (between 4 and 6 kb). All this indicates that mRNAs are transcribed to a significant extent, suggesting that it is now possible, given appropriate nucleotide probes, to analyze the transcription of any given gene during cardiac growth.