Mouse satellite DNA consists of highly repetitive tandem sequences located in the centromeric heterochromatin. It is generally assumed that these simple sequences are not transcribed. We have analyzed total cellular RNA preparations from mouse liver, kidney, brain, and heart tissues at different ages for satellite transcripts. Using recombinant probes containing the major mouse satellite sequence, satellite transcripts were detected only in the heart RNA samples. These transcripts were not detected in the heart muscle of young adult animals (2 and 6 months), but then appeared at the age of 12 months and continued to increase over 2-fold up to the age of 32 months. The transcripts were resistant to DNase I and sensitive to RNases and alkaline treatment. Northern hybridization experiments showed a large and heterogeneous size range of satellite transcripts. Control studies using short-interspersed (B1 and B2) and long-interspersed (L1 and IAP) repetitive DNA sequence probes did not show a similar age-related pattern of transcription. These results indicate that satellite transcription does occur in mice but is highly tissue- and age-specific. The unique occurrence of satellite transcription only in adult and senescent heart tissue indicates age to be an important determinant of gene activity. An understanding of the regulatory mechanisms involved could lead to new insights in the biological role of satellite DNA, gene derepression of reiterated DNA sequences, and the aging processes of cardiac muscle.