Previous work demonstrated that collagen deposition in the liver of rats fed a nutritionally deficient diet for 3 to 4 months was diminished markedly by 16,16-dimethyl prostaglandin E2 treatment. In this study, rats were fed a high-fat diet or a high-fat diet deficient in lipotropes for 2 to 4 weeks prior to liver perfusion. Rates of O2 uptake by the liver were not changed by dietary manipulation. Infusion of 16,16-dimethyl prostaglandin E2 (10 μM), however, decreased O2 uptake by the whole organ by 20 to 40% in both groups. O2 tension was measured at the liver surface with a miniature O2 electrode placed alternatively on periportal and pericentral regions of the liver lobule. Mean O2 tensions in both periportal and pericentral regions were reduced 2- to 3-fold during the infusion of 16,16-dimethyl prostaglandin E2 suggesting an action on the microcirculation. This hypothesis was supported by the observation that fluorescein isothiocyanate-dextran fluorescence detected from the liver surface as well as hepatic vascular volume determined by dye dilution techniques were decreased 30 to 50% by 16,16-dimethyl prostaglandin E2. In addition, 16,16-dimethyl prostaglandin E2 increased portal pressure by about 10 mm Hg in a reversible manner. Thus, it is concluded that pharmacological levels of 16,16-dimethyl prostaglandin E2 affects the microcirculation dramatically in the isolated perfused liver.