This study was carried out to investigate the influence of the membrane fatty acid composition on the basal electrical and contractile activities and the response to beta-adrenergic stimulation of rat cardiac muscle cells in culture. Cells were grown for 3 days in a conventional serum culture medium and then incubated for 24 h in synthetic media containing either n-6 or n-3 as the sole source of polyunsaturated fatty acids (PUFA). The n-6/n-3 ratio in the phospholipids was 0.9 in the n-3 cells and 13.1 in the n-6 cells compared with 6.3 in controls cells. Such modifications did not alter action potentials and the main parameters related to contraction, although shortening was slightly accelerated in the n-6 cells. On the other hand, the positive chronotropic effect induced by isoproterenol was more pronounced (P less than 0.01) in n-3 cells than in n-6 cells. In addition, isoproterenol caused a decrease in contraction duration and in shortening and relaxation time that was less pronounced in n-6 than in control cells (P less than 0.01, P less than 0.01 and P less than 0.05, respectively). These results suggest that the PUFA balance in the phospholipids may contribute to modulate the cardiac adrenergic receptor system but not the membrane properties related to electro-mechanical functions.