Aromatase gene expression and activity have been studied in human skeletal muscle. Using two separate rounds of RT-nested PCR, transcripts from the coding region of aromatase mRNA were detected in 32 of 34 samples. In terms of the non-coding exon I, PCR product for I.4 was detected in 13 cases (38%), I.3 in 10 cases (29%), P.II in 6 cases (18%) and I.1 was not detected in any case. No transcripts for any studied variants of exon I were detected in 18 samples (53%). Aromatase activity was assessed using two different methodologies: in 19 cases by definitive product isolation (DPI) and in 42 cases by tritium-release assay (TRA). Using both methods detectable activity was present in 52% of cases. Average values for the cases with detectable activity were 2.2fmol/mg protein/h for DPI and 6.5fmol/mg protein/h for TRA. In the cohort studied by TRA, a positive correlation of aromatase activity with age of the donor was observed (r=0.34, P=0.03). In six cases paired comparison of aromatase activity in muscle and associated fat tissue were performed by DPI. When expressed per milligram of protein the activity was always higher in fat. However, this difference disappeared when activity was based on the gram of wet tissue. Taking into account bulk in the body it is concluded that muscle can be an important source of estrogens in men and post-menopausal women and its contribution to the circulating pool of estrogens may be comparable to that of adipose tissue. The nature of mRNA transcripts in muscle suggests that estrogen formation may be controlled by glucocorticoid- as well as cAMP-dependent promoters of the aromatase gene.