The serum level of the transforming growth factor-beta1 (TGF-beta1) is elevated after acute bouts of exercise and prolonged training, as well as after myocardial infarction. However, the source of this increase remains unclear. Contracting skeletal muscles are known to be the source of many cytokines. To determine whether skeletal or heart muscles produce TGF-beta1 during exercise, we investigated the effect of a single bout of acute exercise on TGF-beta1 generation in skeletal and heart muscles in untrained rats (UT, n=30) and in rats subjected to prolonged (6-week) endurance training (T, n=29). The UT and T (a day after final training) groups were subjected to an acute bout of exercise with the same work load. Rats from both groups were sacrificed and skeletal and heart muscle samples were collected before (pre), immediately after (0 h), or 3 hours (3 h) after acute exercise. TGF-beta1 mRNA was quantified by RT-PCR in these samples, and basal TGF-beta1 protein levels were determined in skeletal muscle in the UTpre and Tpre subgroups by ELISA. Acute exercise caused a non-significant increase in TGF-beta1 mRNA in skeletal muscle in UT0h rats, in compare to UTpre rats. There was a significant decrease of TGF-beta1 mRNA in the T0h group (p=0.0013) in compare to Tpre rats. Prolonged training caused a significant increase in TGF-beta1 mRNA (p=0.02); however, the TGF-beta1 protein level decreased (p=0.02). In heart muscle, there was a significant decrease of TGF-beta1 mRNA in UT0h (p=0.01) and UT3h (p=0.04) compared to UTpre rats. TGF-beta1 mRNA levels were unchanged in T0h and T3h compared to Tpre; basal TGF-beta1 mRNA expression after training was also unchanged (UTpre vs. Tpre). We conclude that physical exercise is a potent stimulus for inducing TGF-beta1 gene expression in skeletal muscle, but does not increase the protein level. Thus, skeletal and heart muscle do not contribute to increased serum levels of TGF-beta1 after physical exercise.