Japanese medaka (Oryzias latipes) is a valuable model organism in reproductive and developmental toxicity testing. The purpose of this experiment is to assess the response of medaka to aquatic estrogen exposure over the course of 1 year. Each week, three pairs of adult male medaka were exposed separately for 4 days (100% static renewal daily) to 17beta-estradiol at a nominal level of 25 microg/l, with a fourth pair of fish exposed separately to an ethanol control. Vitellogenin (VTG) induction was observed each week, with hepatic and plasma VTG levels significantly higher (P < 0.001) than reported for ethanol control specimens. A significant (P < 0.001) increasing trend was observed for plasma VTG results over the duration of the study, whereas a decreasing trend (P = 0.030) of hepatic VTG was evident. A Durbin-Watson test, however, did not demonstrate any serial autocorrelation of hepatic (d = 1.180) or plasma (d = 1.311) VTG levels over the duration of the study. Time-series transformations of the hepatic and plasma VTG data did not reveal any significant seasonal or behavioral patterns. However, significant intermittent peaks in VTG production were observed in both tissue types during the study. These data indicate that some consideration must be taken to time long-term medaka exposures (>20 weeks) in order to eliminate any influence of cyclic changes on plasma VTG response. Alternatively, hepatic cytosolic measurement of VTG appears to show a more sensitive response to aquatic estrogen exposure.