The short- and long-term effects of sub-lethal exposure to crude oils or an oil-dispersant emulsion on the reproductive success of Leach's Storm-petrels (Oceanodroma leucorhoa) were examined during a three-year field study conducted at a breeding colony in Newfoundland, Canada. Adult petrels were captured in their breeding burrows during the pre-laying, pre-hatching, and post-hatching periods. The birds were individually banded, assigned to either control or experimental groups that received one of a variety of toxicant exposures, and then released back into their permanently marked study burrows. The reproductive success of all study birds was then monitored for the remainder of the breeding season, and the return rates and breeding performance of some groups were monitored during the second breeding season following initial exposure. The results of this investigation indicated: (1) internal or external exposure of adult petrels to relatively high sub-lethal dose levels of crude oils or emulsion significantly reduced hatching success and fledging success in a dose-dependent manner; (2) adult petrels were most sensitive to contaminant exposure late in the incubation period and early in the post-hatching period; (3) pollutant-related decreases in reproductive success were probably associated with the temporary abandonment of the nesting burrow by the treated adult; and (4) treated adults generally exhibited normal return rates and breeding performance in the second season following exposure.