Altered pulmonary function has been reported by many authors in hyperlipemic animals and patients. Our experiments show that isolated lungs of rats rendered hyperlipemic by means of suitable diets display a significantly increased static compliance when pressure-volume (P-V) curves are analyzed during deflation following inflation with air. No differences are observed when P-V curves are obtained with saline. Furthermore the stability index calculated through the analysis of the deflation curve was significantly higher in rats treated with two different hyperlipidic diets (average stability index: 1.06 and 1.04 in the two treated groups, both significantly higher than the two control groups, 0.80 and 0.72, p less than 0.01 in both cases). The observed mechanical effects of hyperlipidemia on lung characteristics in rats show that altered surfactant properties and function play a major role in these experimental conditions.