Rainbow trout Salmo gairdneri, statically exposed to 36 μg/liter anthracene (including 9-14C-anthracene), bioconcentrated the polynuclear aromatic hydrocarbon 200 times the exposure concentration over 18 hours. Then, during a 96-hour clearance period, mass-balance analysis of fish and water samples indicated that anthracene was rapidly converted to polar metabolite(s), then eliminated periodically. Maximum depuration occurred during the dark phase of a 16-hour-light:8-hour-dark photocycle. Of the 2–3% contribution of14C metabolite(s) to the total14C residue, nearly half came from the bile. This periodic depuration may be circadian, although this requires confirmation by further work; to the extent it affects metabolic fate of bioconcentrated organics, periodic depuration undoubtedly contributes to differences between predicted and observed bioconcentration factors.