In the last years there appeared many articles about the adverse influence of non-steroidal anti-inflammatory drugs on the liver and heart. This study is concerned with the influence of the duration of treatment with diclofenac and ketoprofen on the macroscopic and microscopic changes in the liver, lungs, heart, and kidneys in rats. Experiments were carried out on mature Wistar strain rats. Animals of test groups received diclofenac and ketoprofen in a dose of 8 mg/kg/day (equivalent to the therapeutic dose for man) during 7 per os (p.o.) or 28 days intraperitoneally (i.p.), whereas controls received physiological solution p.o. A high morbidity was observed in the animals receiving diclofenac p.o. and somewhat lower in those treated with ketoprofen. On the other hand, the rats got through the 28-day i.p. treatment with both drugs mainly without significant complications. Macroscopic examinations revealed some changes in treated rats: distension of the stomach, ascites, fibrin deposits on the internal organs, lung effusion and the changes in color and structure of the liver. These changes were more frequent in the group of rats receiving diclofenac for the 7 days compared with those that received ketoprofen for the same time. It may be thought that the high mortality and macroscopic changes in the internal organs of experimental animals are a consequence of the microscopic changes in the liver and its lowered function.