Using a primary culture of trout hepatocytes we compare the kinetics of accumulation of vitellogenin and its messenger RNA after estrogen administration. We found that the cells were more sensitive to estradiol than to other estrogens. The lowest effective concentration of estradiol was 10(-9) M. At 10(-6) M the androgens have no effect. Comparison of the primary and secondary stimulation with E2 shows that the initial rate of accumulation of vitellogenin is very much higher in the secondary stimulation. Over a time course of primary stimulation we show that after estradiol withdrawal the rate of accumulation of vitellogenin mRNA in the secondary is a function of time of the first stimulation.