We analyzed desmin and vimentin accumulation in chick myocardiocyte cultures treated with the fibric acid derivatives bezafibrate, fenofibrate and gemfibrozil. The most noteworthy finding was the 50% decrease in the cytoplasmic desmin fraction in cells treated with gemfibrozil in comparison to control cultures, and the 19% increase in the cytoskeletal fraction in cultures treated with gemfibrozil and with bezafibrate. Vimentin accumulation by cells treated with bezafibrate was similar to that in control cultures, however the cytoskeletal vimentin fraction rose by 26% after treatment with gemfibrozil, and fell 13% after treatment with fenofibrate. No vimentin was found in the cytoplasmic fraction of cell treated with bezafibrate. Given the role of intermediate filaments in heart muscle contraction, fibric acid derivative- induced changes in the cytoplasmic and cytoskeletal concentrations of intermediate filament proteins may be related with the secondary effects of these drugs on heart rate.