Herring are commercially valuable, widely exploited fish that spawn in shallow nearshore areas. Because spawning herring attach their eggs to substrates subtidally (less than 10 m) and intertidally, there is potential for the spawning fish and attached eggs to be exposed to spilled oil. Previous studies of the potential effects of oil exposure on hatching success of herring eggs have used static exposure systems with either poorly characterized exposure regimes or ones unlikely to occur in the field. Due to confounding factors, the static exposure systems have often yielded ambiguous results that have not permitted satisfactory assessment of potential oil impacts. Better assessment of the potential impacts requires experimental regimes performed with continuously flowing seawater at exposure concentrations and durations likely to occur in the field. The overall aim of this study was to determine the effects of crude oil on the reproductive success of Pacific herring, Clupea harengus pallasi. Continuously flowing seawater systems provided exposures of appropriate level and duration. Several elements determining reproductive success were examined.