Coal Oil Point (COP) is a natural oil seep off the coast of Santa Barbara, California. Although most studies examining the fate and effects of petroleum have focused upon urbanized or anthropogenic sources of inputs, few have examined the effects of polyaromatic hydrocarbons (PAHs) derived from natural seeps. In order to evaluate the effects of PAHs derived from COP on marine fish populations, hatchery-reared California Halibut (Platichthys californicus) were exposed for 30 days to seven dilutions of sediments collected from COP. Hepatic cytochrome P450 1A (CYP1A), biliary fluorescent aromatic compounds (FACs), gonadal somatic indices, and plasma steroid concentrations. Sixteen USEPA priority PAHs were targeted for analysis in each sediment dilution. In general, biochemical responses were somewhat recalcitrant to dose-response relationships and were less sensitive than the literature values established for the same indicators following exposure to urbanized PAHs. Trends toward reductions in plasma 17beta-estradiol concentrations were observed, but reductions in gonadal somatic indices were not observed. FAC values for naphthalene, benzo(a)pyrene, phenanthrene-related compounds reached maximums at 33-100% COP sediment. The resulting insensitivity may be unique for exposure to "natural" petroleum due to a higher concentration of lower molecular weight PAHs or uncharacterized inhibitors.