This study deals with qualitative alterations of membranes in cardiac cells after chronic inhibition of NO synthesis in rats induced by administration of N(G)-nitro-L-arginine methyl ester (L-NAME) in a dose 40 mg/kg/day for 4 weeks. Concentration of proteins did not change either in the cytosolic fraction or in the particulate fraction of the cardiac homogenate from L-NAME treated rats. The concentration of phospholipids and consequently the ratio of phospholipids to membrane proteins increased by 100% and 88%, respectively. The concentration of conjugated dienes (CD), often used as an indirect marker for the production of free oxygen radicals, increased by 141% after calculation per gram of tissue. However, evaluation of CD concentration directly in phospholipids revealed no change, suggesting that phospholipids in cardiac tissue after L-NAME treatment were not damaged additionally, by increased level of free oxygen radicals. The increase of the CD concentration in the cardiac tissue is therefore, a consequence of the elevated phospholipid concentration.