In assessing adverse health risks from exposure to potential carcinogens, the usual practice is to use cumulative amount or total exposure received over a lifetime as the measure of dose unless there is evidence to the contrary. Different exposure regimens resulting in the same total dose are considered to be equivalent, and the assumption is made that a high dose of a carcinogen received over a short period of time is equivalent to a corresponding low dose spread over a lifetime. Vinyl chloride provides an example for which both toxicologic and pharmacokinetic evidence exists to counter this default assumption. These analyses suggest exposure information that would be useful in assessing health risks when substantial toxicologic and pharmacokinetic information is available.