The effects of pretreatment with a benzodiazepine, diazepam, on the atrial tissue of rats exposed to noise were studied by means of electron microscopy. Examination of ultrathin sections showed that acute stress caused swelling of mitochondria with cristolysis and matrix dissolution. In diazepam-pretreated and noise-exposed animals atrial cardiomyocytes did not reveal noteworthy changes in comparison with controls and the majority of mitochondria were well preserved. Present findings indicate that diazepam is able to exert a protective action on cardiac alterations due to noise exposure, especially at the mitochondrial level.