Estrogen-containing oral contraceptives have been associated with changes in hepatic sinusoids in women. This association has not been studied systematically. To define, quantitate, and locate the estrogen-induced changes within the zones of the hepatic acinus. a morphometric analysis of sinusoids was performed in rat liver. Female rats received oral ethynyl estradiol, dissolved in corn oil, in doses of 0.125 mg/100 g body wt per day for either 2 or 6 wk. Control groups received corn oil in volumes equal to the ethynyl estradiol group or were untreated. After 2 or 6 wk, livers were fixed by perfusion with glutaraldehyde, and 5-microns sections were prepared and analyzed in an automatic image analyzer (Quantimet 720). The ratio of the volume fraction of the sinusoids in zone 1 to that in zone 3 was 2.04 +/- 0.15 (SE) after 2 wk of ethynyl estradiol administration, as compared with 0.87 +/- 0.5 (SE) in rats given corn oil and 0.76 +/- 0.3 (SE) in untreated controls. The differences between the two control groups and the ethynyl estradiol group were significant (P < 0.005). This greater zone 1/zone 3 ratio after ethynyl estradiol was due both to an increased volume fraction of sinusoids in zone 1 as well as to a decreased volume fraction of sinusoids in acinar zone 3, as compared with controls (P < 0.005). The surface to volume ratio of sinusoids, one of the parameters determining the probability of solute-sinusoidal wall interaction, was smaller in zone 1 (P < 0.005) and larger in zone 3 (P < 0.005) in animals treated with ethynyl estradiol than in controls. After 6 wk of ethynyl estradiol administration, the changes in volume fraction persisted. These results indicated that ethynyl estradiol, a synthetic estrogen contained in many oral contraceptives, induced selective zonal changes in rat liver sinusoids. These alterations resulted in the persistent dilatation of sinusoids of acinar zone 1, the acinar inlet, and in the sustained constriction of sinusoids of zone 3, the acinar outlet.