The liver has a great reserve capacity for hepatobiliary bile salt transport. This study was performed to elucidate the significance of this capacity in ethinyl estradiol-induced cholestasis by direct visualization of the zonal involvement in taurocholate transport. The acinar distribution of [3H]taurocholate was determined by histoautoradiographical study of cryopreserved liver slices in normal rats and rats treated with ethinyl estradiol for 5 days. Silver grain densities over the different acinar zones were estimated on digitized image analysis. In control animals, histoautoradiographical study performed 4 min after the start of perfusion showed restriction of taurocholate to acinar zone 1. In contrast, in ethinyl estradiol-treated animals, taurocholate was also found in zone 2 and, in smaller concentrations, in zone 3. In control animals, the relative blackenings by silver grains of acinar zones 1, 2 and 3 were 66% +/- 1.2%, 25% +/- 1.6% and 5% +/- 0.6%, respectively. After 5 days of ethinyl estradiol treatment, blackenings were 58% +/- 1.5%, 36% +/- 2.1% and 12% +/- 0.8%, respectively. As early as 15 sec after injection of [3H]taurocholate, the bile canalicular areas of the cell plates and the bile ductules of ethinyl estradiol-treated animals were labeled as intensely as those of control animals. Our results demonstrate ethinyl estradiol-induced recruitment of the acinar zones 2 and 3 for hepatobiliary taurocholate transport. This recruitment may largely compensate for reduction of transport capacity of periportal hepatocytes in early cholestasis.